Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net fair value of Level 3 transfers	$ 187	[1]
Fuel Oils [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers into Level 3 / Transfers out of Level 1	2	[1]
Transfers out of Level 3 / Transfers into Level 1	(1)	[1]
Natural Gas [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3 / Transfers into Level 2	185	[1]
Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3 / Transfers into Level 2	$ 1	[1]

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