SCHEDULE 13G

Amendment No. 0 
Activision Inc 
Common Stock 
Cusip #004930202 


Cusip #004930202 
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	3,741,734 
Item 6:	0 
Item 7:	23,027,408 
Item 8:	0 
Item 9:	23,027,408 
Item 11:	11.413% 
Item 12:	    HC


Cusip #004930202  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	23,027,408 
Item 8:	0 
Item 9:	23,027,408 
Item 11:	11.413% 
Item 12:	IN 


Cusip #004930202 
Item 1:	Reporting Person - Abigail P. Johnson 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	23,027,408 
Item 8:	0 
Item 9:	23,027,408 
Item 11:	11.413% 
Item 12:	    IN



	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)




Item 1(a).	Name of Issuer:	

		Activision Inc 

Item 1(b).	Name of Issuer's Principal Executive Offices:

		3100 Ocean park Blvd.
		Suite 1000 
		Santa Monica, CA  90405
		  

Item 2(a).	Name of Person Filing: 

		FMR Corp.

Item 2(b).	Address or Principal Business Office or, if None, 
Residence:

		82 Devonshire Street, Boston, 
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock 

Item 2(e).	CUSIP Number:  

		004930202 

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) 
and the person filing, FMR Corp., is a parent holding company 
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See 
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	23,027,408 

	(b)	Percent of Class:	11.413% 

	(c)	Number of shares as to which such 
person has:  

	(i)	sole power to vote or to direct 
the vote:	3,741,734 

	(ii)	shared power to vote or to 
direct the vote:	0 

	(iii)	sole power to dispose or to 
direct the disposition of:	23,027,408 

	(iv)	shared power to dispose or to 
direct the disposition of:	0 



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another 
Person.

	Various persons have the right to receive or the power to 
direct the receipt of dividends from, or the proceeds from the 
sale of, the Common Stock of Activision Inc.  No one person's 
interest in the Common Stock of Activision Inc is more than 
five percent of the total outstanding Common Stock.

Item 7.	Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on By the Parent 
Holding Company.

	See attached Exhibit(s) A, B, and C. 

Item 8.	Identification and Classification of Members of 
the Group.

	Not Applicable. See attached Exhibit A. 

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge 
and belief, the securities referred to above were acquired in the 
ordinary course of business and were not acquired for the 
purpose of and do not have the effect of changing or 
influencing the control of the issuer of such securities and 
were not acquired in connection with or as a participant in any 
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this Schedule 
13G in connection with FMR Corp.'s beneficial ownership of 
the Common Stock of Activision Inc at July 31, 2005 is true, 
complete and correct. 

August 10, 2005 			
Date

/s/Joseph Mari				
Signature

Joseph Mari				
Duly authorized under Power of Attorney
dated July 20, 2005, by Eric D. Roiter by and on behalf of 
FMR Corp. and its direct and indirect subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 
13G, Fidelity Management & Research Company 
("Fidelity"), 82 Devonshire Street, Boston, Massachusetts 
02109, a wholly-owned subsidiary of FMR Corp. and an 
investment adviser registered under Section 203 of the 
Investment Advisers Act of 1940, is the beneficial owner of 
19,288,013 shares or 9.560% of the Common Stock 
outstanding of Activision Inc  ("the Company") as a result of 
acting as investment adviser to various investment companies 
registered under Section 8 of the Investment Company Act of 
1940.

	Edward C. Johnson 3d, FMR Corp., through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 19,288,013 shares owned by the Funds.

	Neither FMR Corp. nor Edward C. Johnson 3d, 
Chairman of FMR Corp., has the sole power to vote or direct 
the voting of the shares owned directly by the Fidelity Funds, 
which power resides with the Funds' Boards of Trustees.  
Fidelity carries out the voting of the shares under written 
guidelines established by the Funds' Boards of Trustees.

	Fidelity Management Trust Company, 82 Devonshire 
Street, Boston, Massachusetts 02109, a wholly-owned 
subsidiary of FMR Corp. and a bank as defined in Section 
3(a)(6) of the Securities Exchange Act of 1934, is the 
beneficial owner of  2,778,649 shares or 1.377% of the 
Common Stock outstanding of the Company as a result of its 
serving as investment manager of the institutional account(s).

	Edward C. Johnson 3d and FMR Corp., through its 
control of Fidelity Management Trust Company, each has 
sole dispositive power over 2,778,649 shares and sole power 
to vote or to direct the voting of 2,778,649 shares of 
Common Stock owned by the institutional account(s) as 
reported above.

	Members of the Edward C. Johnson 3d family are the 
predominant owners of Class B shares of common stock of 
FMR Corp., representing approximately 49% of the voting 
power of FMR Corp.  Mr. Johnson 3d owns 12.0% and 
Abigail Johnson owns 24.5% of the aggregate outstanding 
voting stock of FMR Corp.  Mr. Johnson 3d is Chairman of 
FMR Corp. and Abigail P. Johnson is a Director of FMR 
Corp.  The Johnson family group and all other Class B 
shareholders have entered into a shareholders' voting 
agreement under which all Class B shares will be voted in 
accordance with the majority vote of Class B shares.  
Accordingly, through their ownership of voting common 
stock and the execution of the shareholders' voting 
agreement, members of the Johnson family may be deemed, 
under the Investment Company Act of 1940, to form a 
controlling group with respect to FMR Corp.

	Fidelity International Limited, Pembroke Hall, 42 
Crowlane, Hamilton, Bermuda, and various foreign-based 
subsidiaries provide investment advisory and management 
services to a number of non-U.S. investment companies (the 
"International Funds") and certain institutional investors.  
Fidelity International Limited is the beneficial owner of 
960,746 shares or 0.476% of the Common Stock outstanding 
of the Company. Additional information with respect to the 
beneficial ownership of Fidelity International Limited is 
shown on Exhibit B.

 


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)

	Pursuant to instructions in Item 7 of Schedule 13G, this 
Exhibit has been prepared  to identify Fidelity International 
Limited, Pembroke Hall, 42 Crow Lane, Hamilton, Bermuda, 
a Bermudan joint stock company incorporated for an 
unlimited duration by private act of the Bermuda Legislature 
(FIL) and an investment adviser to various investment 
companies (the "International Funds") and certain institutional 
investors, as a beneficial owner of the 960,746 shares or 
0.476% of the Common Stock outstanding of Activision Inc.

	Prior to June 30, 1980, FIL was a majority-owned 
subsidiary of Fidelity Management & Research Company 
(Fidelity), a wholly-owned subsidiary of FMR Corp.  On that 
date, the shares of FIL held by Fidelity were distributed, as a 
dividend,  to the shareholders of FMR Corp.  FIL currently 
operates as an entity independent of FMR Corp. and Fidelity.  
The International Funds and FIL's other clients, with the 
exception of Fidelity and an affiliated company of Fidelity, are 
non-U.S. entities.

	A partnership controlled by Edward C. Johnson 3d and 
members of his family owns shares of FIL voting stock with 
the right to cast approximately 39.89% of the total votes which 
may be cast by all holders of FIL voting stock.  Mr. Johnson 
3d is Chairman of FMR Corp. and FIL.  FMR Corp. and FIL 
are separate and independent corporate entities, and their 
Boards of Directors are generally composed of different 
individuals.  Other than when one serves as a sub adviser to 
the other, their investment decisions are made independently, 
and their clients are generally different organizations.

	FMR Corp. and FIL are of the view that they are not 
acting as a "group" for purposes of Section 13(d) under the 
Securities Exchange Act of 1934 (the "1934" Act) and that 
they are not otherwise required to attribute to each other the 
"beneficial ownership" of securities "beneficially owned" by 
the other corporation within the meaning of Rule 13d-3 
promulgated under the 1934 Act.  Therefore, they are of the 
view that the shares held by the other corporation need not be 
aggregated for purposes of Section 13(d). However, FMR 
Corp. is making this filing on a voluntary basis as if all of the 
shares are beneficially owned by FMR Corp. and FIL on a 
joint basis.

	FIL may continue to have the International Funds or 
other accounts purchase shares subject to a number of factors, 
including, among others, the availability of shares for sale at 
what FIL considers to be reasonable prices and other 
investment opportunities that may be available to the 
International Funds.

	FIL intends to review continuously the equity position 
of the International Funds and other accounts in the Company.  
Depending upon its future evaluations of the business and 
prospects of the Company and upon other developments, 
including, but not limited to, general economic and business 
conditions and money market and stock market conditions, 
FIL may determine to cease making additional purchases of 
shares or to increase or decrease the equity interest in the 
Company by acquiring additional shares, or by disposing of all 
or a portion of the shares.

	FIL does not have a present plan or proposal which 
relates to or would result in (i) an extraordinary corporate 
transaction, such as a merger, reorganization, liquidation, or 
sale or transfer of a material amount of assets involving the 
Company or any of its subsidiaries, (ii) any change in the 
Company's present Board of Directors or management, (iii) 
any material changes in the Company's present capitalization 
or dividend policy or any other material change in the 
Company's business or corporate structure, (iv) any change in 
the Company's charter or by-laws, or (v) the Company's 
common stock becoming eligible for termination of its 
registration pursuant to Section 12(g)(4) of the 1934 Act.

	FIL  has sole power to vote and the sole power to 
dispose of  960,746  shares.


 	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on August 10, 2005, agree 
and consent to the joint filing on their behalf of this Schedule 
13G in connection with their beneficial ownership of the 
Common Stock of Activision Inc at July 31, 2005.

	FMR Corp.

	By /s/ Joseph Mari			
	Joseph Mari
	Duly authorized under Power of Attorney
	dated July 20, 2005, by Eric D. Roiter by and on behalf 
of FMR Corp. and its direct and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Joseph Mari			
	Joseph Mari
	Duly authorized under Power of Attorney
	dated July 20, 2005, by Eric D. Roiter by and on behalf 
of Edward C. Johnson 3d

	Abigail P. Johnson

	By /s/ Joseph Mari			
	Joseph Mari
	Duly authorized under Power of Attorney
	dated July 20, 2005, by Eric D. Roiter by and on behalf 
of Abigail P. Johnson

	Fidelity Management & Research Company

	By /s/ Joseph Mari			
	Joseph Mari
	Duly authorized under Power of Attorney dated July 
20, 2005, by Eric D. Roiter Secretary