Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Natural Gas [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3	[1]
Natural Gas [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]
Counterparty credit risk	0.26%	[3],[4]
Nodal basis	(0.18)	[3]
Credit risk	2.00%	[3],[4]
Natural Gas [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.46%	[2]
Counterparty credit risk	5.00%	[3],[4]
Nodal basis	0	[3]
Credit risk	3.00%	[3],[4]
Natural Gas [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.35%	[2]
Counterparty credit risk	1.00%	[3],[4]
Nodal basis	(0.01)	[3]
Credit risk	3.00%	[3],[4]
Natural Gas [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]
Nodal basis	(0.24)	[3]
Natural Gas [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.46%	[2]
Nodal basis	(0.04)	[3]
Natural Gas [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.37%	[2]
Nodal basis	(0.19)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	2.00%	[3],[4]
Natural Gas [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]
Counterparty credit risk	0.26%	[3],[4]
Nodal basis	(0.18)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.46%	[2]
Counterparty credit risk	5.00%	[3],[4]
Nodal basis	(0.02)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.26%	[2]
Counterparty credit risk	1.00%	[3],[4]
Nodal basis	(0.04)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%
Nodal basis	(0.24)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.46%
Nodal basis	(0.04)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.37%
Nodal basis	(0.19)	[3]
Natural Gas [Member] | Union Electric Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[1]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[3],[4]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]
Counterparty credit risk	0.70%	[3]
Nodal basis	(0.05)	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.46%	[2]
Counterparty credit risk	2.00%	[3]
Nodal basis	0	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.33%	[2]
Counterparty credit risk	1.00%	[3]
Nodal basis	0	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	2	[1]
Natural Gas [Member] | Derivative Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[1]
Natural Gas [Member] | Derivative Liabilities [Member] | Union Electric Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[1]
Natural Gas [Member] | Derivative Liabilities [Member] | Ameren Illinois Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		[1]
Fuel Oils [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	7	[1]	8	[1]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]	0.21%	[2]
Counterparty credit risk	0.26%	[3],[4]	0.12%	[3],[4]
Credit risk			2.00%	[3],[4]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.59%	[2]	0.60%	[2]
Counterparty credit risk	3.00%	[3],[4]	1.00%	[3],[4]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.39%	[2]	0.44%	[2]
Counterparty credit risk	1.00%	[3],[4]	1.00%	[3],[4]
Fuel Oils [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatalities	10.00%	[2]	7.00%	[2]
Fuel Oils [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatalities	19.00%	[2]	27.00%	[2]
Fuel Oils [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatalities	17.00%	[2]	24.00%	[2]
Fuel Oils [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk			2.00%	[3],[4]
Fuel Oils [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.20%	[2]	0.21%	[2]
Counterparty credit risk	0.26%	[3],[4]	0.12%	[3],[4]
Fuel Oils [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.59%	[2]	0.60%	[2]
Counterparty credit risk	3.00%	[3],[4]	1.00%	[3],[4]
Fuel Oils [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	0.39%	[2]	0.44%	[2]
Counterparty credit risk	1.00%	[3],[4]	1.00%	[3],[4]
Fuel Oils [Member] | Union Electric Company [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatalities	10.00%	[2]	7.00%	[2]
Fuel Oils [Member] | Union Electric Company [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatalities	19.00%	[2]	27.00%	[2]
Fuel Oils [Member] | Union Electric Company [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatalities	17.00%	[2]	24.00%	[2]
Fuel Oils [Member] | Union Electric Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	7	[1]	8	[1]
Fuel Oils [Member] | Derivative Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(2)	[1]	(3)	[1]
Fuel Oils [Member] | Derivative Liabilities [Member] | Union Electric Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(2)	[1]	(3)	[1]
Power [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[3],[4]	0.22%	[3],[4]
Nodal basis	(5)	[3]	(5)	[3]
Credit risk	2.00%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and offpeak pricing	24	[3]	22	[3]
Estimated auction price	0	[2]	(281)	[2]
Power [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[3],[4]	1.00%	[3],[4]
Nodal basis	(1)	[3]	(1)	[3]
Credit risk	3.00%	[3],[4]	5.00%	[3],[4]
Average bid/ask consensus peak and offpeak pricing	48	[3]	47	[3]
Estimated auction price	4,280	[2]	1,851	[2]
Power [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[3],[4]	1.00%	[3],[4]
Nodal basis	(3)	[3]	(3)	[3]
Credit risk	3.00%	[3],[4]	5.00%	[3],[4]
Average bid/ask consensus peak and offpeak pricing	33	[3]	31	[3]
Estimated auction price	190	[2]	178	[2]
Power [Member] | Fundamental Energy Production Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	5	[2]	4	[2]
Power [Member] | Fundamental Energy Production Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	7	[2]	8	[2]
Power [Member] | Fundamental Energy Production Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	6	[2]	6	[2]
Power [Member] | Contract Price Allocation [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4]	5	[2]
Power [Member] | Contract Price Allocation [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4]	7	[2]
Power [Member] | Contract Price Allocation [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4]	6	[2]
Power [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	12	[1],[5]	14	[1],[5]
Power [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	2.00%	[3],[4]	2.00%	[3],[4]
Power [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[3],[4]	0.22%	[3],[4]
Nodal basis	(5)	[3]	(5)	[3]
Average bid/ask consensus peak and offpeak pricing	24	[3]	24	[3]
Estimated auction price	0	[2]	(281)	[2]
Power [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[3],[4]	1.00%	[3],[4]
Nodal basis	(1)	[3]	(1)	[3]
Average bid/ask consensus peak and offpeak pricing	51	[3]	56	[3]
Estimated auction price	4,280	[2]	1,851	[2]
Power [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[3],[4]	1.00%	[3],[4]
Nodal basis	(3)	[3]	(2)	[3]
Average bid/ask consensus peak and offpeak pricing	38	[3]	36	[3]
Estimated auction price	190	[2]	178	[2]
Power [Member] | Union Electric Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	9	[1],[5]	14	[1],[5]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[3],[4]	5.00%	[3],[4]
Credit risk	3.00%	[3],[4]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Nodal basis	(4)	[2]	(5)	[2]
Average bid/ask consensus peak and offpeak pricing	24	[2]	22	[2]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Nodal basis	(1)	[2]	(1)	[2]
Average bid/ask consensus peak and offpeak pricing	42	[2]	47	[2]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Nodal basis	(3)	[2]	(3)	[2]
Average bid/ask consensus peak and offpeak pricing	32	[2]	30	[2]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	5	[2]	4	[2]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	7	[2]	8	[2]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated future gas prices	6	[2]	6	[2]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[2]	5	[2]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[2]	7	[2]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[2]	6	[2]
Power [Member] | Ameren Illinois Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3	[1],[5]	0	[1],[5]
Power [Member] | Derivative Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(91)	[1],[5]	(114)	[1],[5]
Power [Member] | Derivative Liabilities [Member] | Union Electric Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(7)	[1],[5]	(3)	[1],[5]
Power [Member] | Derivative Liabilities [Member] | Ameren Illinois Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(84)	[1],[5]	(111)	[1],[5]
Uranium [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	42	[2]	43	[2]
Uranium [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	45	[2]	46	[2]
Uranium [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	43	[2]	44	[2]
Uranium [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]		[1]
Uranium [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	42	[2]	43	[2]
Uranium [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	45	[2]	46	[2]
Uranium [Member] | Union Electric Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	43	[2]	44	[2]
Uranium [Member] | Union Electric Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]		[1]
Uranium [Member] | Derivative Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(2)	[1]	(2)	[1]
Uranium [Member] | Derivative Liabilities [Member] | Union Electric Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(2)	[1]	(2)	[1]

[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 higher (lower) fair value measurement.

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

[4] 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.

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