SCHEDULE 13G

Amendment No. 4 
Agere Systems Incorporated 
Class A Common Stock 
Cusip #00845V100 


Cusip #00845V100 
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	641,909 
Item 6:	0 
Item 7:	81,274,780 
Item 8:	0 
Item 9:	81,274,780 
Item 11:	9.773% 
Item 12:	    HC


Cusip #00845V100  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	81,274,780 
Item 8:	0 
Item 9:	81,274,780 
Item 11:	9.773% 
Item 12:	IN 


Cusip #00845V100 
Item 1:	Reporting Person - Abigail P. Johnson 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	81,274,780 
Item 8:	0 
Item 9:	81,274,780 
Item 11:	9.773% 
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:	

		Agere Systems Incorporated 

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

		555 Union Boulevard  
		Allentown, PA  18109
		  

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:  

		00845V100 

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:	81,274,780 

	(b)	Percent of Class:	9.773% 

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

	(i)	sole power to vote or to direct 
the vote:	641,909 

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

	(iii)	sole power to dispose or to 
direct the disposition of:	81,274,780 

	(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 Class A Common Stock of Agere Systems 
Incorporated.  No one person's interest in the Class A 
Common Stock of Agere Systems Incorporated is more than 
five percent of the total outstanding Class A 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. 

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 Class A Common Stock of Agere Systems Incorporated at 
December 31, 2004 is true, complete and correct. 

February 14, 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)



	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 
80,698,342 shares or 9.703% of the Class A Common Stock 
outstanding of Agere Systems Incorporated  ("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. The number of shares of 
Class A Common Stock of Agere Systems Incorporated 
owned by the investment companies at December 31, 2004 
included 6,179,895 shares of Class A Common Stock 
resulting from the assumed conversion of $20,440,000 
principal amount of AGERE SYS CONV 6.5% 12/15/09 
(302.3432 shares of Class A Common Stock for each $1,000 
principal amount of debenture).

	Edward C. Johnson 3d, FMR Corp., through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 80,698,342 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  572,600 shares or 0.069% 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 572,600 shares and sole power to 
vote or to direct the voting of 572,600 shares of Class A 
Common Stock owned by the institutional account(s) as 
reported above.

	Strategic Advisers, Inc., 82 Devonshire Street, 
Boston, MA 02109, a wholly-owned subsidiary of FMR 
Corp. and an investment adviser registered under Section 203 
of the Investment Advisers Act of 1940, provides investment 
advisory services to individuals.  As such, FMR Corp.'s 
beneficial ownership includes 3,838 shares, or 0.000%, of the 
Class A Common Stock stock outstanding of Agere Systems 
Incorporated, beneficially owned through Strategic Advisers, 
Inc.

	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.

 


	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 February 14, 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 Agere Systems Incorporated at 
December 31, 2004.

	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

	Abigail P. Johnson

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Abigail P. Johnson

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Senior V.P. and General Counsel