Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	$ (112)	[1]	$ 51	[1]
Fuel Oils [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(15)	[1]		[1]
Natural Gas [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	84	[1]	(26)	[1]
Power [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(180)	[1]	80	[1]
Uranium [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(1)	[1]	(3)	[1]
Ameren Missouri [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(15)		15
Ameren Missouri [Member] | Fuel Oils [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(15)
Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	10
Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(9)		18
Ameren Missouri [Member] | Uranium [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	(1)		(3)
Ameren Illinois [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	103		186
Ameren Illinois [Member] | Natural Gas [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	74		(26)
Ameren Illinois [Member] | Power [Member]
Derivative [Line Items]
Gain (Loss) RecognizedIn Regulatory Liabilitiesor Regulatory Assets	$ 29		$ 212

[1] Includes amounts for intercompany eliminations.