Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 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]	0.22%	0.40%
Nodal basis	[3]	(0.80)	(0.10)
Credit risk	[1],[2]	0.38%	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]	0.38%	0.40%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	2.00%	7.00%
Nodal basis	[3]	(0.50)	(0.10)
Credit risk	[1],[2]	0.38%	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 Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]	$ (1)	$ (1)
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.56%	0.86%
Nodal basis | $ / MWh	[5]	(9)	(10)
Credit risk	[1],[2]	0.38%	0.40%
Average forward peak and off-peak pricing | $ / MWh	[5]	27	22
Estimated auction price | $ / MW	[3]	(309)	(270)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.56%	0.86%
Nodal basis | $ / MWh	[5]	(1)	(1)
Credit risk	[1],[2]	0.38%	0.40%
Average forward peak and off-peak pricing | $ / MWh	[5]	43	39
Estimated auction price | $ / MW	[3]	1,509	2,057
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.56%	0.86%
Nodal basis | $ / MWh	[5]	(2)	(3)
Credit risk	[1],[2]	0.38%	0.40%
Average forward peak and off-peak pricing | $ / MWh	[5]	30	29
Estimated auction price | $ / MW	[3]	96	211
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[3],[6]	4.00%	3.00%
Estimated future gas prices	[3]	3	3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[3],[6]	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],[6]	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	[3]	5	5
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[3]	7	7
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[3]	6	6
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4],[7]	$ 15	$ 16
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4],[7]	$ (170)	$ (170)
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.38%	0.40%
Average forward pricing | $ / lb	[3]	27	35
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.38%	0.40%
Average forward pricing | $ / lb	[3]	30	42
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.38%	0.40%
Average forward pricing | $ / lb	[3]	29	37
Uranium | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]	21.00%	20.00%
Uranium | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]	21.00%	20.00%
Uranium | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]	21.00%	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] The derivative asset and liability balances are presented net of counterparty credit considerations.

[5] 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.

[6] Escalation rate applies to power prices in 2031 and beyond for June 30, 2016 and to power prices in 2026 and beyond for December 31, 2015.

[7] 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.