Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 equipment agreement		Dec. 31, 2011 agreement equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	$ 23,909	[1],[2]	$ 26,468	[1],[2]
Accumulated depreciation and amortization	8,823	[1],[2]	9,429	[1],[2]
Property and plant, before construction work in progress	15,086	[1],[2]	17,039	[1],[2]
Property, Plant and Equipment, Net	16,096	[1],[2]	18,127	[1],[2]
Number of combustion turbine electric generation equipment under capital lease agreements	2		2
Number of capital lease agreements	2		2
Capital lease agreements, gross asset value	228		229
Total accumulated depreciation, capital lease agreements	52		52
Held-to-maturity Securities	304		309
Electric [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	22,055	[1],[2]	24,717	[1],[2]
Gas [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,854	[1],[2]	1,751	[1],[2]
Nuclear Fuel [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	317	[1],[2]	255	[1],[2]
Other Energy [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	693	[1],[2]	833	[1],[2]
Union Electric Company [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	16,031	[2]	15,484	[2]
Accumulated depreciation and amortization	6,614	[2]	6,276	[2]
Property and plant, before construction work in progress	9,417	[2]	9,208	[2]
Property, Plant and Equipment, Net	10,161	[2]	9,958	[2]
Union Electric Company [Member] | Electric [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	15,638	[2]	15,099	[2]
Union Electric Company [Member] | Gas [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	393	[2]	385	[2]
Union Electric Company [Member] | Nuclear Fuel [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	317	[2]	255	[2]
Union Electric Company [Member] | Other Energy [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	427	[2]	495	[2]
Ameren Illinois Company [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,446		6,052
Accumulated depreciation and amortization	1,495		1,364
Property and plant, before construction work in progress	4,951		4,688
Property, Plant and Equipment, Net	5,052		4,770
Ameren Illinois Company [Member] | Electric [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	4,985		4,684
Ameren Illinois Company [Member] | Gas [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,461		1,368
Ameren Illinois Company [Member] | Other Energy [Member]
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 101		$ 82

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries as well as intercompany eliminations.

[2] Amounts in Ameren and Ameren Missouri include two electric generation CTs under two separate capital lease agreements. The gross asset value of those agreements was $228 million and $229 million at December 31, 2012, and 2011, respectively. The total accumulated depreciation associated with the two CTs was $52 million and $52 million at December 31, 2012, and 2011, respectively. In addition, Ameren Missouri has investments in debt securities, which are classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2012, and 2011, the carrying value of these debt securities was $304 million and $309 million, respectively.