Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	$ 56	[1]	$ (112)	[1]
Fuel Oils
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(2)		(15)
Natural Gas
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	52		84
Power
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	10	[1]	(180)	[1]
Uranium
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(4)		(1)
Union Electric Company
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	5		(15)
Union Electric Company | Fuel Oils
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(2)		(15)
Union Electric Company | Natural Gas
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	4		10
Union Electric Company | Power
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	7		(9)
Union Electric Company | Uranium
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(4)		(1)
Ameren Illinois Company
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	51		103
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	48		74
Ameren Illinois Company | Power
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	$ 3		$ 29

[1] Amounts include intercompany eliminations