Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 134	[1]	$ 103	[1]	$ 81	[1]
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	45	[1]	129	[1]	36	[1]
Union Electric Company [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	52	[1]	43	[1]	36	[1]
Union Electric Company [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	9	[1]	9	[1]	11	[1]
Ameren Illinois Company [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	46		28		23
Ameren Illinois Company [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	35		118		20
Other [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	36		32		22
Other [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 1		$ 2		$ 5

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