Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer out of Level 3/Transfers out of Level 2		[1]		[1]
Net fair value of Level 3 transfers	183	[1]	(31)	[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]		[1]
Transfers out of Level 3/Transfers into Level1	(1)	[1]	(30)	[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]		[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	(3)	[1]	(1)	[1]
Union Electric Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer out of Level 3/Transfers out of Level 2			(1)
Net fair value of Level 3 transfers	12		(19)
Union Electric Company [Member] | 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	1
Transfers out of Level 3/Transfers into Level1			(19)
Union Electric Company [Member] | 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	15
Union Electric Company [Member] | Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3/Transfers into Level 2	(4)		1
Ameren Illinois Company [Member] | 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	$ 170

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