Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net fair value of Level 3 transfers	$ 1	$ 182
Fuel Oils
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers into Level 3/Transfers out of Level 1		1
Natural Gas
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3/Transfers into Level 2		185
Power
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer into Level 3/Transfers out of Level 2	(3)
Transfers out of Level 3/Transfers into Level 2	4	(4)
Union Electric Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net fair value of Level 3 transfers	1	12
Union Electric Company | Fuel Oils
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers into Level 3/Transfers out of Level 1		1
Union Electric Company | Natural Gas
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 | Power
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer into Level 3/Transfers out of Level 2	(3)
Transfers out of Level 3/Transfers into Level 2	4	(4)
Ameren Illinois Company | Natural Gas
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3/Transfers into Level 2		$ 170