Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Natural Gas [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 2	[1]
Natural Gas [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[2],[3]
Natural Gas [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[2],[3]
Nodal basis	(0.1)	[3]
Natural Gas [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[2],[3]
Nodal basis	0	[3]
Natural Gas [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[2],[3]
Nodal basis	0	[3]
Natural Gas [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatilities	1.00%	[4]
Nodal basis	(0.350)	[3]
Natural Gas [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatilities	31.00%	[4]
Nodal basis	(0.06)	[3]
Natural Gas [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatilities	24.00%	[4]
Nodal basis	(0.3)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[2],[3]
Natural Gas [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[2],[3]
Nodal basis	(0.1)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[2],[3]
Nodal basis	0	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[2],[3]
Nodal basis	(0.10)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate
Volatilities	1.00%	[4]
Nodal basis	(0.35)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate
Volatilities	31.00%	[4]
Nodal basis	(0.06)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate
Volatilities	24.00%	[4]
Nodal basis	(0.30)	[3]
Natural Gas [Member] | Ameren Missouri [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[2],[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.69%	[2],[3]
Nodal basis	(0.1)	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[2],[3]
Nodal basis	0	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[2],[3]
Nodal basis	0	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatilities	1.00%	[4]
Nodal basis	(0.3)	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatilities	31.00%	[4]
Nodal basis	(0.27)	[3]
Natural Gas [Member] | Ameren Illinois Company [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatilities	27.00%	[4]
Nodal basis	(0.28)	[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] | Ameren Missouri [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]
Fair Value Inputs [Abstract]
Credit risk			2.00%	[2],[3]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.21%	[4]
Counterparty credit risk	0.26%	[2],[3]	0.12%	[2],[3]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.60%	[4]
Counterparty credit risk	3.00%	[2],[3]	1.00%	[2],[3]
Fuel Oils [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.44%	[4]
Counterparty credit risk	2.00%	[2],[3]	1.00%	[2],[3]
Fuel Oils [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatilities	8.00%	[4]	7.00%	[4]
Fuel Oils [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatilities	32.00%	[4]	27.00%	[4]
Fuel Oils [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatilities	20.00%	[4]	24.00%	[4]
Fuel Oils [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk			2.00%	[2],[3]
Fuel Oils [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.21%	[4]
Counterparty credit risk	0.26%	[2],[3]	0.12%	[2],[3]
Fuel Oils [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.60%	[4]
Counterparty credit risk	3.00%	[2],[3]	1.00%	[2],[3]
Fuel Oils [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate			0.44%	[4]
Counterparty credit risk	2.00%	[2],[3]	1.00%	[2],[3]
Fuel Oils [Member] | Ameren Missouri [Member] | Option Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatilities	8.00%	[4]	7.00%	[4]
Fuel Oils [Member] | Ameren Missouri [Member] | Option Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatilities	32.00%	[4]	27.00%	[4]
Fuel Oils [Member] | Ameren Missouri [Member] | Option Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Volatilities	20.00%	[4]	24.00%	[4]
Fuel Oils [Member] | Ameren Missouri [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	(4)	[1]	(3)	[1]
Fuel Oils [Member] | Derivative Liabilities [Member] | Ameren Missouri [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(4)	[1]	(3)	[1]
Power [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[2],[3]
Power [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[2],[3]	0.22%	[2],[3]
Nodal basis	(4)	[3]	(5)	[3]
Credit risk			2.00%	[2],[3]
Average bid/ask consensus peak and offpeak pricing	25	[3]	22	[3]
Estimated auction price	(767)	[4]	(281)	[4]
Power [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	7.00%	[2],[3]	1.00%	[2],[3]
Nodal basis	(1)	[3]	(1)	[3]
Credit risk			5.00%	[2],[3]
Average bid/ask consensus peak and offpeak pricing	49	[3]	47	[3]
Estimated auction price	1,790	[4]	1,851	[4]
Power [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	3.00%	[2],[3]	1.00%	[2],[3]
Nodal basis	(3)	[3]	(3)	[3]
Credit risk			5.00%	[2],[3]
Average bid/ask consensus peak and offpeak pricing	32	[3]	31	[3]
Estimated auction price	252	[4]	178	[4]
Power [Member] | Fundamental Energy Production Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[4],[5]
Estimated future gas prices	5	[4]	4	[4]
Power [Member] | Fundamental Energy Production Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[4],[5]
Estimated future gas prices	8	[4]	8	[4]
Power [Member] | Fundamental Energy Production Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[4],[5]
Estimated future gas prices	6	[4]	6	[4]
Power [Member] | Contract Price Allocation [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4]	5	[4]
Power [Member] | Contract Price Allocation [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4]	7	[4]
Power [Member] | Contract Price Allocation [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4]	6	[4]
Power [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	44	[1],[6]	14	[1],[6]
Power [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Credit risk	3.00%	[2],[3]	2.00%	[2],[3]
Power [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	0.22%	[2],[3]	0.22%	[2],[3]
Nodal basis	(4)	[3]	(5)	[3]
Average bid/ask consensus peak and offpeak pricing	25	[3]	24	[3]
Estimated auction price	(767)	[4]	(281)	[4]
Power [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	3.00%	[2],[3]	1.00%	[2],[3]
Nodal basis	(1)	[3]	(1)	[3]
Average bid/ask consensus peak and offpeak pricing	49	[3]	56	[3]
Estimated auction price	1,790	[4]	1,851	[4]
Power [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	3.00%	[2],[3]	1.00%	[2],[3]
Nodal basis	(2)	[3]	(2)	[3]
Average bid/ask consensus peak and offpeak pricing	38	[3]	36	[3]
Estimated auction price	252	[4]	178	[4]
Power [Member] | Ameren Missouri [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	42	[1],[6]	14	[1],[6]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member]
Fair Value Inputs [Abstract]
Counterparty credit risk	7.00%	[2],[3]
Credit risk	3.00%	[2],[3]	5.00%	[2],[3]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Nodal basis	(4)	[4]	(5)	[4]
Average bid/ask consensus peak and offpeak pricing	26	[4]	22	[4]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Nodal basis	(1)	[4]	(1)	[4]
Average bid/ask consensus peak and offpeak pricing	39	[4]	47	[4]
Power [Member] | Ameren Illinois Company [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Nodal basis	(3)	[4]	(3)	[4]
Average bid/ask consensus peak and offpeak pricing	30	[4]	30	[4]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[4],[5]
Estimated future gas prices	5	[4]	4	[4]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[4],[5]
Estimated future gas prices	8	[4]	8	[4]
Power [Member] | Ameren Illinois Company [Member] | Fundamental Energy Production Model [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[4],[5]
Estimated future gas prices	6	[4]	6	[4]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4]	5	[4]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4]	7	[4]
Power [Member] | Ameren Illinois Company [Member] | Contract Price Allocation [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4]	6	[4]
Power [Member] | Ameren Illinois Company [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	2	[1],[6]		[1],[6]
Power [Member] | Derivative Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(87)	[1],[6]	(114)	[1],[6]
Power [Member] | Derivative Liabilities [Member] | Ameren Missouri [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(5)	[1],[6]	(3)	[1],[6]
Power [Member] | Derivative Liabilities [Member] | Ameren Illinois Company [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(82)	[1],[6]	(111)	[1],[6]
Uranium [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[4]	43	[4]
Uranium [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	44	[4]	46	[4]
Uranium [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[4]	44	[4]
Uranium [Member] | Derivative Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[1]		[1]
Uranium [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[4]	43	[4]
Uranium [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	44	[4]	46	[4]
Uranium [Member] | Ameren Missouri [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[4]	44	[4]
Uranium [Member] | Ameren Missouri [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	(3)	[1]	(2)	[1]
Uranium [Member] | Derivative Liabilities [Member] | Ameren Missouri [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ (3)	[1]	$ (2)	[1]

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

[2] 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.

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

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

[5] Escalation rate applies to power prices 2026 and beyond.

[6] 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.