Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) (USD $)	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Other Debt Securities [Member]
Fair value of plan assets					$ 1,000,000
Real Estate [Member]
Fair value of plan assets	108,000,000		98,000,000		90,000,000
Private Equity [Member]
Fair value of plan assets	23,000,000		28,000,000		33,000,000
Pension Benefits [Member]
Fair value of plan assets	2,876,000,000	[1]	2,722,000,000	[1]	2,495,000,000	[1]
Pension Benefits [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	389,000,000
Pension Benefits [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	2,424,000,000
Pension Benefits [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Fair value of plan assets	131,000,000
Pension Benefits [Member] | Cash And Cash Equivalents [Member]
Fair value of plan assets	31,000,000		20,000,000
Pension Benefits [Member] | Cash And Cash Equivalents [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	31,000,000		20,000,000
Pension Benefits [Member] | U.S. Large Capitalization [Member]
Fair value of plan assets	994,000,000		882,000,000
Pension Benefits [Member] | U.S. Large Capitalization [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	72,000,000		70,000,000
Pension Benefits [Member] | U.S. Large Capitalization [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	922,000,000		812,000,000
Pension Benefits [Member] | U.S. Small And Mid Capitalization [Member]
Fair value of plan assets	213,000,000		309,000,000
Pension Benefits [Member] | U.S. Small And Mid Capitalization [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	202,000,000		299,000,000
Pension Benefits [Member] | U.S. Small And Mid Capitalization [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	11,000,000		10,000,000
Pension Benefits [Member] | International And Emerging Markets [Member]
Fair value of plan assets	328,000,000		413,000,000
Pension Benefits [Member] | International And Emerging Markets [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	115,000,000		129,000,000
Pension Benefits [Member] | International And Emerging Markets [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	213,000,000		284,000,000
Pension Benefits [Member] | Corporate Bonds [Member]
Fair value of plan assets	720,000,000		646,000,000
Pension Benefits [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	720,000,000		646,000,000
Pension Benefits [Member] | Municipal Bonds [Member]
Fair value of plan assets	176,000,000		129,000,000
Pension Benefits [Member] | Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	176,000,000		129,000,000
Pension Benefits [Member] | U.S. Treasury And Agency Securities [Member]
Fair value of plan assets	230,000,000		154,000,000
Pension Benefits [Member] | U.S. Treasury And Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	230,000,000		154,000,000
Pension Benefits [Member] | Other Debt Securities [Member]
Fair value of plan assets	121,000,000		100,000,000
Pension Benefits [Member] | Other Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	121,000,000		100,000,000
Pension Benefits [Member] | Real Estate [Member]
Fair value of plan assets	108,000,000		98,000,000
Pension Benefits [Member] | Real Estate [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Fair value of plan assets	108,000,000		98,000,000
Pension Benefits [Member] | Private Equity [Member]
Fair value of plan assets	23,000,000		28,000,000
Pension Benefits [Member] | Private Equity [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Fair value of plan assets	23,000,000		28,000,000
Pension Benefits [Member] | Derivative Assets [Member]
Fair value of plan assets	1,000,000		1,000,000
Pension Benefits [Member] | Derivative Assets [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	1,000,000		1,000,000
Pension Benefits [Member] | Derivative Liabilities [Member]
Fair value of plan assets	(1,000,000)		(1,000,000)
Pension Benefits [Member] | Derivative Liabilities [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	(1,000,000)		(1,000,000)
Pension Benefits [Member] | Medical Benefit Component Excluded [Member]
Fair value of plan assets	(91,000,000)	[2]	(85,000,000)	[2]
Pension Benefits [Member] | Pending Security Sales Included [Member]
Fair value of plan assets	23,000,000	[3]	28,000,000	[3]
Postretirement Benefits [Member]
Fair value of plan assets	896,000,000	[1]	797,000,000	[1]	732,000,000	[1]
Postretirement Benefits [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	337,000,000		324,000,000
Postretirement Benefits [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	501,000,000		394,000,000
Postretirement Benefits [Member] | Cash And Cash Equivalents [Member]
Fair value of plan assets	67,000,000		35,000,000
Postretirement Benefits [Member] | Cash And Cash Equivalents [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	1,000,000
Postretirement Benefits [Member] | Cash And Cash Equivalents [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	66,000,000		35,000,000
Postretirement Benefits [Member] | U.S. Large Capitalization [Member]
Fair value of plan assets	313,000,000		287,000,000
Postretirement Benefits [Member] | U.S. Large Capitalization [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	235,000,000		215,000,000
Postretirement Benefits [Member] | U.S. Large Capitalization [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	78,000,000		72,000,000
Postretirement Benefits [Member] | U.S. Small And Mid Capitalization [Member]
Fair value of plan assets	57,000,000		66,000,000
Postretirement Benefits [Member] | U.S. Small And Mid Capitalization [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	57,000,000		66,000,000
Postretirement Benefits [Member] | International [Member]
Fair value of plan assets	100,000,000		94,000,000
Postretirement Benefits [Member] | International [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets	44,000,000		43,000,000
Postretirement Benefits [Member] | International [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	56,000,000		51,000,000
Postretirement Benefits [Member] | Corporate Bonds [Member]
Fair value of plan assets	61,000,000		59,000,000
Postretirement Benefits [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	61,000,000		59,000,000
Postretirement Benefits [Member] | Municipal Bonds [Member]
Fair value of plan assets	86,000,000		58,000,000
Postretirement Benefits [Member] | Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	86,000,000		58,000,000
Postretirement Benefits [Member] | U.S. Treasury And Agency Securities [Member]
Fair value of plan assets	82,000,000		59,000,000
Postretirement Benefits [Member] | U.S. Treasury And Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	82,000,000		59,000,000
Postretirement Benefits [Member] | Asset-Backed Securities [Member]
Fair value of plan assets	23,000,000		31,000,000
Postretirement Benefits [Member] | Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	23,000,000		31,000,000
Postretirement Benefits [Member] | Other Debt Securities [Member]
Fair value of plan assets	49,000,000		29,000,000
Postretirement Benefits [Member] | Other Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets	49,000,000		29,000,000
Postretirement Benefits [Member] | Medical Benefit Component Excluded [Member]
Fair value of plan assets	91,000,000	[2]	85,000,000	[2]
Postretirement Benefits [Member] | Pending Security Sales Excluded [Member]
Fair value of plan assets	(33,000,000)	[4]	(6,000,000)	[4]
Includes Medical Benefit Component Under Section 401(h) And Excludes Payables Related To Pending Security Sales [Member] | Postretirement Benefits [Member]
Fair value of plan assets	896,000,000		797,000,000
Excludes Medical Benefit Component Under Section 401(h) And Includes Receivables Related To Pending Security Sales [Member] | Pension Benefits [Member]
Fair value of plan assets	2,876,000,000
Excludes Medical Benefit Component Under Section 401(h) And Includes Receivables Related To Pending Security Sales [Member] | Pension Benefits [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Fair value of plan assets			498,000,000
Excludes Medical Benefit Component Under Section 401(h) And Includes Receivables Related To Pending Security Sales [Member] | Pension Benefits [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair value of plan assets			2,155,000,000
Excludes Medical Benefit Component Under Section 401(h) And Includes Receivables Related To Pending Security Sales [Member] | Pension Benefits [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Fair value of plan assets			126,000,000
Excludes Medical Benefit Component Under Section 401(h) And Excludes Payables Related To Pending Security Sales [Member] | Postretirement Benefits [Member]
Fair value of plan assets	838,000,000		718,000,000
Includes Medical Benefit Component Under Section 401(h) And Excludes Receivables Related To Pending Security Sales [Member] | Pension Benefits [Member]
Fair value of plan assets	$ 2,944,000,000		$ 2,779,000,000

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code (401(h) accounts) to fund a portion of the postretirement obligation.

[3] Receivables related to pending security sales, offset by payables related to pending security purchases.

[4] Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.