Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 equipment		Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	26,198	[1]	$ 23,701	[1]
Accumulated depreciation and amortization	9,759	[1]	8,644	[1]
Property and plant, before construction work in progress	16,439	[1]	15,057	[1]
Property, Plant and Equipment, Net	17,424	[1]	16,205	[1]
Number of combustion turbine electric generation equipment under capital lease agreements	2
Number of capital lease agreements	2
Capital lease agreements, gross asset value	233		228
Total accumulated depreciation, capital lease agreements	66		56
Held-to-maturity Securities	294		299
Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	23,913	[1]	21,726	[1]
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,285	[1]	1,975	[1]
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	209	[1]	246	[1]
Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	776	[1]	902	[1]
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	17,483	[1]	16,377	[1]
Accumulated depreciation and amortization	7,086	[1]	6,766	[1]
Property and plant, before construction work in progress	10,397	[1]	9,611	[1]
Property, Plant and Equipment, Net	10,867	[1]	10,452	[1]
Union Electric Company | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	17,052	[1]	15,964	[1]
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	431	[1]	413	[1]
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	209	[1]	246	[1]
Union Electric Company | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	261	[1]	595	[1]
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	8,371		6,988
Accumulated depreciation and amortization	2,422		1,627
Property and plant, before construction work in progress	5,949		5,361
Property, Plant and Equipment, Net	6,165		5,589
Ameren Illinois Company | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,517		5,426
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,854		1,562
Ameren Illinois Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	0		0
Ameren Illinois Company | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	216		228
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	344		336
Accumulated depreciation and amortization	251		251
Property and plant, before construction work in progress	93		85
Property, Plant and Equipment, Net	392		164
Other | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	344		336
Other | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0		0
Other | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	0		0
Other | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	299		$ 79

[1] (a)Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $228 million at December 31, 2014 and 2013, respectively. The total accumulated depreciation associated with the two CTs was $66 million and $56 million at December 31, 2014 and 2013, respectively. In addition, Ameren Missouri has investments in debt securities, which were classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2014 and 2013, the carrying value of these debt securities was $294 million and $299 million, respectively.