Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2016 USD ($) $ / credit $ / MMBTU $ / lb $ / MWh $ / MW		Dec. 31, 2015 USD ($) $ / credit $ / MMBTU $ / lb $ / MWh $ / MW
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.40%		0.40%
Nodal basis	[3]	(0.60)		(0.10)
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	6.00%		12.00%
Nodal basis	[3]	0		0
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.93%		7.00%
Nodal basis	[3]	(0.50)		(0.10)
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]			35.00%
Nodal basis	[2]			(0.30)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]			55.00%
Nodal basis	[2]			0
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]			45.00%
Nodal basis	[2]			(0.20)
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0		$ 1
Derivative liabilities | $	[4]	$ (1)		$ (1)
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(10)	[5]	(10)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	18		22
Estimated auction price | $ / MW	[3]	(818)		(270)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(1)	[5]	(1)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	37		39
Estimated auction price | $ / MW	[3]	1,062		2,057
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(2)	[5]	(3)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	27		29
Estimated auction price | $ / MW	[3]	67		211
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	3		3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	5		4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	4		4
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		5	[5]	5	[3]
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		7	[5]	7	[3]
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		6	[5]	6	[3]
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4],[8]	$ 6		$ 16
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4],[8]	$ (187)		$ (170)
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	28		35
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	33		42
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	31		37
Uranium | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0		$ 0
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]	$ (4)		$ (1)

[1] 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.

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

[3] 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] Not applicable.

[6] 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.

[7] Escalation rate applies to power prices 2031 and beyond for March 31, 2016 and to power prices 2026 and beyond for December 31, 2015.

[8] Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand.