SCHEDULE 13G

Amendment No. 4 
Cumulus Media Incorporated 
Class A Common Stock 
Cusip #231082108 


Cusip #231082108 
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	732,000 
Item 6:	0 
Item 7:	732,000 
Item 8:	0 
Item 9:	732,000 
Item 11:	1.414% 
Item 12:	    HC


Cusip #231082108  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	732,000 
Item 8:	0 
Item 9:	732,000 
Item 11:	1.414% 
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:	

		Cumulus Media Incorporated 

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

		3535 Piedmont Road
		Building 14, Fourteenth Floor 
		Atlanta, GA  30305
		  

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:

		Class A Common Stock 

Item 2(e).	CUSIP Number:  

		231082108 

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:	732,000 

	(b)	Percent of Class:	1.414% 

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

	(i)	sole power to vote or to direct 
the vote:	732,000 

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

	(iii)	sole power to dispose or to 
direct the disposition of:	732,000 

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



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

	If this statement is being filed to report the fact that as of the 
date hereof, the reporting person has ceased to be the 
beneficial owner of more than five percent of the class of 
securities, check the following (X).

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

	Not applicable

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

	See attached Exhibit A. 

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.

	Inasmuch as the reporting persons are no longer the beneficial 
owners of more than five percent of the number of shares 
outstanding, the reporting persons have no further reporting 
obligation under Section 13(d) of the Securities and Exchange 
Commission thereunder, and the reporting persons have no 
obligation to amend this Statement if any material change 
occurs in the facts set forth herein.

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this statement 
is true, complete and correct. 

December 12, 2005 			
Date

/s/Eric D. Roiter			
Signature

Eric D, Roiter			
Duly authorized under Power of Attorney
dated December 30, 1997 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)



	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  732,000 shares or 1.414% of the Class A 
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 732,000 shares and sole power to vote 
or to direct the voting of 732,000 shares of Class A Common 
Stock owned by the institutional account(s) as reported above.

	Members of the family of Edward C. Johnson 3d, 
Chairman of FMR Corp., are the predominant owners, directly 
or through trusts, of Series B shares of common stock of FMR 
Corp., representing 49% of the voting power of FMR Corp.  
The Johnson family group and all other Series B shareholders 
have entered into a shareholders' voting agreement under 
which all Series B shares will be voted in accordance with the 
majority vote of Series 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.



	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 December 12, 2005, agree 
and consent to the joint filing on their behalf of this Schedule 
13G in connection with their beneficial ownership of the Class 
A Common Stock of Cumulus Media Incorporated at 
November 30, 2005.

	FMR Corp.

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney dated 
December 30, 1997, by and on behalf of FMR Corp. and its 
direct and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney dated 
December 30, 1997, by and on behalf of Edward C. Johnson 
3d