Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Details) (Commodity Contracts [Member], USD $) In Millions, except Per Share data, unless otherwise specified																	3 Months Ended																																												3 Months Ended								3 Months Ended
	Mar. 31, 2012 Fuel Oils [Member]		Dec. 31, 2011 Fuel Oils [Member]		Mar. 31, 2012 Power [Member]		Dec. 31, 2011 Power [Member]		Mar. 31, 2012 Uranium [Member]		Dec. 31, 2011 Uranium [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Option Model [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Option Model [Member] Uranium [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Option Model [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Option Model [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Option Model [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Credit Risk Discount [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Credit Risk Discount [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Credit Risk Discount [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Forward/Swaps Third Party Pricing [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Forward/Swaps Third Party Pricing [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Forward/Swaps Third Party Pricing [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Basis To Nodal Valuation Price [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Basis To Nodal Valuation Price [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Basis To Nodal Valuation Price [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Market Simulation Model [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Market Simulation Model [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Power Market Simulation Model [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Contract Price Allocation [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Contract Price Allocation [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Contract Price Allocation [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Third Party Pricing [Member] Minimum [Member] Uranium [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Third Party Pricing [Member] Maximum [Member] Uranium [Member]		Mar. 31, 2012 Derivative Liabilities [Member] Third Party Pricing [Member] Weighted Average [Member] Uranium [Member]		Mar. 31, 2012 Derivative Assets [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Escalated Exchange Settled Pricing [Member] Minimum [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Escalated Exchange Settled Pricing [Member] Maximum [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Escalated Exchange Settled Pricing [Member] Weighted Average [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Minimum [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Maximum [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Weighted Average [Member] Fuel Oils [Member]		Mar. 31, 2012 Derivative Assets [Member] Option Model [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Minimum [Member] Fuel Oils [Member]	Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Minimum [Member] Power [Member]	Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Maximum [Member] Fuel Oils [Member]	Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Maximum [Member] Power [Member]	Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Weighted Average [Member] Fuel Oils [Member]	Mar. 31, 2012 Derivative Assets [Member] Credit Risk Discount [Member] Weighted Average [Member] Power [Member]	Mar. 31, 2012 Derivative Assets [Member] Power Forward/Swaps Third Party Pricing [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Power Forward/Swaps Third Party Pricing [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Power Forward/Swaps Third Party Pricing [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] FTR Third Party Pricing [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] FTR Third Party Pricing [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] FTR Third Party Pricing [Member] Weighted Average [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Basis To Nodal Valuation Price [Member] Minimum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Basis To Nodal Valuation Price [Member] Maximum [Member] Power [Member]		Mar. 31, 2012 Derivative Assets [Member] Basis To Nodal Valuation Price [Member] Weighted Average [Member] Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 45	[1],[2]	$ 37	[1],[2]	$ 185	[1],[2]	$ 195	[1],[2]																																																			$ 170	[1],[2],[3]							$ 9	[1],[2]
Derivative liabilities	$ 1	[1],[2]	$ 2	[1],[2]	$ 210	[1],[2]	$ 80	[1],[2]	$ 1	[1],[2]	$ 1	[1],[2]	$ 194	[1],[2],[3]	$ 1	[1],[2],[3]
Escalation rate																																																													0.68%	[4]	0.71%	[4]	0.71%	[4]
Volatilities																	15.00%	[5]	40.00%	[5]	24.00%	[5]																																															23.00%	[4]	15.00%	[5]	28.00%	[4]	68.00%	[5]	25.00%	[4]	19.00%	[5]
Average bid/ask consensus pricing																	$ 16	[5]	$ 39	[5]	$ 34	[5]							$ 20	[5]	$ 49	[5]	$ 28	[5]																			$ 51	[4]	$ 55	[4]	$ 52	[4]													$ 16	[5]			$ 39	[5]			$ 35	[5]							$ 20	[5]	$ 49	[5]	$ 29	[5]
Counterparty credit risk																																																																																	0.12%	0.06%	12.00%	13.00%	1.00%	5.00%
Estimated auction price																																																																																													$ (1,569)	[4]	$ 3,019	[4]	$ 173	[4]
Nodal basis																																			$ (6)	[4]	$ (0.20)	[4]	$ (3)	[4]																																																											$ (6)	[4]	$ (0.20)	[4]	$ (3)	[4]
Estimated future gas prices																																									$ 4	[4]	$ 6	[4]	$ 5	[4]
Estimated renewable energy credit costs																																															$ 5	[4]	$ 7	[4]	$ 6	[4]
Ameren credit risk																							3.00%	[5]	6.00%	[5]	6.00%	[5]

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

[2] The derivative asset and liability balances are presented net of counterparty credit considerations.

[3] Power valuations utilize visible third party pricing evaluated by month for peak and off-peak through 2015. Valuations beyond 2015 utilize power market simulation modeled pricing by month for peak and off-peak.

[4] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

[5] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.