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, 2015		Dec. 31, 2014
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.31%	[1],[2]	0.43%	[1],[2]
Nodal basis	(0.40)	[3]	(0.40)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	12.07%	[1],[2]	13.00%	[1],[2]
Nodal basis	(0.1)	[3]	0.10	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	2.47%	[1],[2]	3.00%	[1],[2]
Nodal basis	(0.30)	[3]	(0.20)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	6.00%	[3]	31.00%	[1]
Nodal basis	(0.40)	[3]	(0.40)	[3]
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	39.00%	[3]	144.00%	[1]
Nodal basis	(0.20)	[3]	0	[3]
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	33.00%	[3]	63.00%	[1]
Nodal basis	(0.30)	[3]	(0.20)	[3]
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 1	[4]	$ 1	[4]
Natural Gas | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.54%	[1],[2]	0.57%	[1],[2]
Nodal basis	(0.1)	[3]	(0.10)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	12.07%	[1],[2]	13.00%	[1],[2]
Nodal basis	(0.1)	[3]	(0.10)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[1],[2]	5.00%	[1],[2]
Nodal basis	(0.1)	[3]	(0.10)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Union Electric Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	6.00%	[3]	31.00%	[1]
Nodal basis	(0.40)	[3]	(0.40)	[3]
Natural Gas | Union Electric Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	39.00%	[3]	144.00%	[1]
Nodal basis	(0.20)	[3]	0	[3]
Natural Gas | Union Electric Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	33.00%	[3]	53.00%	[1]
Nodal basis	(0.30)	[3]	(0.30)	[3]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.31%	[1],[2]	0.43%	[1],[2]
Nodal basis	(0.4)	[3]	(0.40)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	2.28%	[1],[2]	2.00%	[1],[2]
Nodal basis	(0.1)	[3]	0.10	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	1.28%	[1],[2]	0.83%	[1],[2]
Nodal basis	(0.3)	[3]	(0.20)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas | Ameren Illinois Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities			50.00%	[1]
Nodal basis			(0.10)	[3]
Natural Gas | Ameren Illinois Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities			144.00%	[1]
Nodal basis			0	[3]
Natural Gas | Ameren Illinois Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities			94.00%	[1]
Nodal basis			(0.10)	[3]
Natural Gas | Ameren Illinois Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	1	[4]
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[4]	(2)	[4]
Natural Gas | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[4]	(1)	[4]
Natural Gas | Derivative Liabilities | Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities			(1)	[4]
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	33.00%	[1]	3.00%	[1]
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	80.00%	[1]	39.00%	[1]
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	42.00%	[1]	32.00%	[1]
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	2	[4]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils | Union Electric Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	33.00%	[1]	3.00%	[1]
Fuel Oils | Union Electric Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	80.00%	[1]	39.00%	[1]
Fuel Oils | Union Electric Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	42.00%	[1]	32.00%	[1]
Fuel Oils | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	2	[4]
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(7)	[4]	(8)	[4]
Fuel Oils | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(7)	[4]	(8)	[4]
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.29%	[1],[2]	0.26%	[1],[2]
Nodal basis	(11)	[3]	(6)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	22	[6]	27	[7]
Estimated auction price	(597)	[3]	(1,833)	[3]
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	10.98%	[1],[2]	0.26%	[1],[2]
Nodal basis	0	[3]	0	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	46	[6]	50	[7]
Estimated auction price	1,922	[3]	2,743	[3]
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.84%	[1],[2]	0.26%	[1],[2]
Nodal basis	(3)	[3]	(2)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	32	[6]	32	[7]
Estimated auction price	153	[3]	171	[3]
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	0.00%	[3],[9]
Estimated future gas prices	3	[3]	4	[3]
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	5	[3]	5	[3]
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	4	[3]	4	[3]
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[3]	5	[3]
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[3]	7	[3]
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[3]	6	[3]
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	5	[10],[4]	11	[11],[4]
Power | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.29%	[1],[2]	0.26%	[1],[2]
Nodal basis	(11)	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	24	[1]	27	[1]
Estimated auction price	(597)	[3]	(1,833)	[3]
Power | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	10.98%	[1],[2]	0.26%	[1],[2]
Nodal basis	(4)	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	46	[1]	50	[1]
Estimated auction price	1,922	[3]	2,743	[3]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.84%	[1],[2]	0.26%	[1],[2]
Nodal basis	(8)	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	36	[1]	32	[1]
Estimated auction price	153	[3]	171	[3]
Power | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	5	[10],[4]	11	[11],[4]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Nodal basis	(6)	[3]	(6)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	22	[3]
Average forward pricing			27	[3]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Nodal basis	0	[3]	0	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	38	[3]
Average forward pricing			38	[3]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Nodal basis	(3)	[3]	(2)	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	31	[3]
Average forward pricing			32	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	0.00%	[3],[9]
Estimated future gas prices	3	[3]	4	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	5	[3]	5	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	4	[3]	4	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[3]	5	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[3]	7	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[3]	6	[3]
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(165)	[10],[4]	(144)	[11],[4]
Power | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[10],[4]	(2)	[11],[4]
Power | Derivative Liabilities | Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(164)	[10],[4]	(142)	[11],[4]
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	35	[3]
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing	43	[3]	40	[3]
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	36	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	35	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing	43	[3]	40	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	36	[3]
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(1)	[4]	(2)	[4]
Uranium | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ (1)	[4]	$ (2)	[4]

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

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

[4] (a)The derivative asset and liability balances are presented net of counterparty credit considerations.

[5] (f)Escalation rate applies to fuel oil prices 2017 and beyond.

[6] (g)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

[7] (h)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

[8] (h)Escalation rate applies to power prices 2026 and beyond.

[9] (i)Escalation rate applies to power prices 2026 and beyond.

[10] (f)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.

[11] (g)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.