Property And Plant, Net (Accrued Capital Expenditures) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 181	[1]	$ 175	[1]	$ 107	[1]
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	72		74		63
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	59		86		37
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	13	[1]	8	[1]	8	[1]
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 13		$ 8		$ 8

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