Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Fuel Oils [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets	9	[1]
Discounted Cash Flow [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Credit risk	2.00%	[2],[3]
Discounted Cash Flow [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Credit risk	2.00%	[2],[3],[4]
Discounted Cash Flow [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[2],[3],[4]
Discounted Cash Flow [Member] | Minimum [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.21%	[5]
Counterparty credit risk	0.12%	[2],[3]
Credit risk	2.00%	[2],[3]
Discounted Cash Flow [Member] | Minimum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.04%	[2],[3],[4]
Credit risk	2.00%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	16	[2],[4]
Estimated auction price for FTRs	(133,787)	[4],[5]
Nodal basis	(12)	[2],[4]
Discounted Cash Flow [Member] | Minimum [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	43	[5]
Discounted Cash Flow [Member] | Minimum [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.21%	[5]
Counterparty credit risk	0.12%	[2],[3]
Discounted Cash Flow [Member] | Minimum [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	24	[2],[4]
Estimated auction price for FTRs	(281)	[4],[5]
Nodal basis	(5)	[2],[4]
Discounted Cash Flow [Member] | Minimum [Member] | Union Electric Company [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	43	[5]
Discounted Cash Flow [Member] | Minimum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus peak and off-peak pricing	22	[4],[5]
Nodal basis	(5)	[4],[5]
Discounted Cash Flow [Member] | Maximum [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.68%	[5]
Counterparty credit risk	1.00%	[2],[3]
Credit risk	31.00%	[2],[3]
Discounted Cash Flow [Member] | Maximum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	100.00%	[2],[3],[4]
Credit risk	5.00%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	52	[2],[4]
Estimated auction price for FTRs	19,671	[4],[5]
Nodal basis	1	[2],[4]
Discounted Cash Flow [Member] | Maximum [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	46	[5]
Discounted Cash Flow [Member] | Maximum [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.60%	[5]
Counterparty credit risk	1.00%	[2],[3]
Discounted Cash Flow [Member] | Maximum [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	56	[2],[4]
Estimated auction price for FTRs	1,851	[4],[5]
Nodal basis	(1)	[2],[4]
Discounted Cash Flow [Member] | Maximum [Member] | Union Electric Company [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	46	[5]
Discounted Cash Flow [Member] | Maximum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus peak and off-peak pricing	47	[4],[5]
Nodal basis	(1)	[4],[5]
Discounted Cash Flow [Member] | Weighted Average [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.48%	[5]
Counterparty credit risk	1.00%	[2],[3]
Credit risk	12.00%	[2],[3]
Discounted Cash Flow [Member] | Weighted Average [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[2],[3],[4]
Credit risk	5.00%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	32	[2],[4]
Estimated auction price for FTRs	198	[4],[5]
Nodal basis	(1)	[2],[4]
Discounted Cash Flow [Member] | Weighted Average [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	44	[5]
Discounted Cash Flow [Member] | Weighted Average [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.44%	[5]
Counterparty credit risk	1.00%	[2],[3]
Discounted Cash Flow [Member] | Weighted Average [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[2],[3],[4]
Average bid/ask consensus peak and off-peak pricing	36	[2],[4]
Estimated auction price for FTRs	178	[4],[5]
Nodal basis	(2)	[2],[4]
Discounted Cash Flow [Member] | Weighted Average [Member] | Union Electric Company [Member] | Uranium [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	44	[5]
Discounted Cash Flow [Member] | Weighted Average [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus peak and off-peak pricing	30	[4],[5]
Nodal basis	(3)	[4],[5]
Option Model [Member] | Minimum [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	7.00%	[5]
Option Model [Member] | Minimum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Volatilities	13.00%	[2],[4]
Average bid/ask consensus peak and off-peak pricing	24	[2],[4]
Option Model [Member] | Minimum [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	7.00%	[5]
Option Model [Member] | Maximum [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	27.00%	[5]
Option Model [Member] | Maximum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Volatilities	38.00%	[2],[4]
Average bid/ask consensus peak and off-peak pricing	45	[2],[4]
Option Model [Member] | Maximum [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	27.00%	[5]
Option Model [Member] | Weighted Average [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	24.00%	[5]
Option Model [Member] | Weighted Average [Member] | Power [Member]
Fair Value Inputs [Abstract]
Volatilities	26.00%	[2],[4]
Average bid/ask consensus peak and off-peak pricing	36	[2],[4]
Option Model [Member] | Weighted Average [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs [Abstract]
Volatilities	24.00%	[5]
Fundamental Energy Production Model [Member] | Minimum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	4	[4],[5]
Fundamental Energy Production Model [Member] | Minimum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	4	[4],[5]
Fundamental Energy Production Model [Member] | Maximum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	8	[4],[5]
Fundamental Energy Production Model [Member] | Maximum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	8	[4],[5]
Fundamental Energy Production Model [Member] | Weighted Average [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	6	[4],[5]
Fundamental Energy Production Model [Member] | Weighted Average [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	6	[4],[5]
Contract Price Allocation [Member] | Minimum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4],[5]
Contract Price Allocation [Member] | Minimum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4],[5]
Contract Price Allocation [Member] | Maximum [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4],[5]
Contract Price Allocation [Member] | Maximum [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4],[5]
Contract Price Allocation [Member] | Weighted Average [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4],[5]
Contract Price Allocation [Member] | Weighted Average [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4],[5]
Derivative liabilities [Member] | Fuel Oils [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	3	[1]
Derivative liabilities [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	172	[1]
Derivative liabilities [Member] | Uranium [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	2	[1]
Derivative liabilities [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	3	[1]
Derivative liabilities [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	3	[1]
Derivative liabilities [Member] | Union Electric Company [Member] | Uranium [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	2	[1]
Derivative liabilities [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative liabilities	111	[1],[4]
Derivative assets [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets	131	[1]
Derivative assets [Member] | Uranium [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]
Derivative assets [Member] | Union Electric Company [Member] | Fuel Oils [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets	8	[1]
Derivative assets [Member] | Union Electric Company [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets	14	[1]
Derivative assets [Member] | Union Electric Company [Member] | Uranium [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]
Derivative assets [Member] | Ameren Illinois Company [Member] | Power [Member]
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative assets		[1],[4]

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

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

[3] Counterparty credit risk is only applied to counterparties with derivative asset balances. Ameren, Ameren Missouri, and Ameren Illinois credit risk is only applied to counterparties with derivative liability balances.

[4] Power valuations utilize visible third party pricing evaluated by month for peak and off-peak through 2017. Valuations beyond 2017 utilize fundamentally modeled pricing by month for peak and off-peak.

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