Property And Plant, Net (Accrued Capital Expenditures) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 108	[1]	$ 107	[1]	$ 79	[1]
Union Electric Company [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	63		73		53
Ameren Illinois Company [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 37		$ 18		$ 15

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