Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.30%	[1],[2]
Nodal basis	(0.1)	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[1],[2]
Nodal basis	0	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.62%	[1],[2]
Nodal basis	(0.1)	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 1	[3]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.30%	[1],[2]
Nodal basis	(0.1)	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[1],[2]
Nodal basis	0	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.62%	[1],[2]
Nodal basis	(0.1)	[1]
Credit risk	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[3]
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[1],[2]	2.00%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.72%	[1],[2]	1.00%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	2.00%	[4]	10.00%	[4]
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	27.00%	[4]	35.00%	[4]
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	14.00%	[4]	16.00%	[4]
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3	[3]	8	[3]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	1.00%	[1],[2]	2.00%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.72%	[1],[2]	1.00%	[1],[2]
Credit risk	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	2.00%	[4]	10.00%	[4]
Fuel Oils | Union Electric Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	27.00%	[4]	35.00%	[4]
Fuel Oils | Union Electric Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	14.00%	[4]	16.00%	[4]
Fuel Oils | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3	[3]	8	[3]
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(3)	[3]	(3)	[3]
Fuel Oils | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(3)	[3]	(3)	[3]
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.39%	[1],[2]
Nodal basis	(6)	[1]	(3)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	29	[1]	25	[1]
Estimated auction price	(1,853)	[4]	(1,594)	[4]
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.50%	[1],[2]
Nodal basis	0	[1]	(1)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	59	[1]	51	[1]
Estimated auction price	2,087	[4]	945	[4]
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.42%	[1],[2]
Nodal basis	(3)	[1]	(2)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	35	[1]	32	[1]
Estimated auction price	199	[4]	305	[4]
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	3.00%	[4],[5]
Estimated future gas prices	4	[4]	4	[4]
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4]	5	[4]
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4]	7	[4]
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4]	6	[4]
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	10	[3],[6]	21	[3],[7]
Power | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.39%	[1],[2]
Nodal basis			(3)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	30	[1]	25	[1]
Estimated auction price	(1,853)	[4]	(1,594)	[4]
Power | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.50%	[1],[2]
Nodal basis			(1)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	59	[1]	51	[1]
Estimated auction price	2,087	[4]	945	[4]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.40%	[1],[2]	0.42%	[1],[2]
Nodal basis			(2)	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	48	[1]	40	[1]
Estimated auction price	199	[4]	305	[4]
Power | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	10	[3],[6]	21	[3],[7]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Nodal basis	(6)	[4]	(4)	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	29	[4]	27	[4]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Nodal basis	0	[4]	0	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	42	[4]	36	[4]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Nodal basis	(3)	[4]	(2)	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	33	[4]	30	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	3.00%	[4],[5]
Estimated future gas prices	4	[4]	4	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[4]	5	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[4]	7	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[4]	6	[4]
Power | Ameren Illinois Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets				[3],[7]
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(129)	[3],[6]	(110)	[3],[7]
Power | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(5)	[3],[6]	(2)	[3],[7]
Power | Derivative Liabilities | Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(124)	[3],[6]	(108)	[3],[7]
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing	35	[4]	34	[4]
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing	41	[4]	41	[4]
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	36	[4]	36	[4]
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[3]		[3]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing	35	[4]	34	[4]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing	41	[4]	41	[4]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	36	[4]	36	[4]
Uranium | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[3]		[3]
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(3)	[3]	(6)	[3]
Uranium | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ (3)	[3]	$ (6)	[3]

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

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

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

[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 use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

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