Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Discounted Cash Flow | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.21%	[1]
Counterparty credit risk	0.26%	[2],[3]	0.12%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Minimum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.39%	[2],[3]	0.22%	[2],[3]
Credit risk	2.00%	[2],[3]	2.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	25	[3]	22	[3]
Estimated auction price for FTRs	(1,594)	[1]	(281)	[1]
Nodal basis	(3)	[3]	(5)	[3]
Discounted Cash Flow | Minimum | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	34	[1]	43	[1]
Discounted Cash Flow | Minimum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.21%	[1]
Counterparty credit risk	0.26%	[2],[3]	0.12%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Minimum | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.39%	[2],[3]	0.22%	[2],[3]
Credit risk	2.00%	[2],[3]	2.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	25	[3]	24	[3]
Estimated auction price for FTRs	(1,594)	[1]	(281)	[1]
Nodal basis	(3)	[3]	(5)	[3]
Discounted Cash Flow | Minimum | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	34	[1]	43	[1]
Discounted Cash Flow | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	2.00%	[2],[3]	5.00%	[2],[3]
Nodal basis	(4)	[1]	(5)	[1]
Average bid/ask consensus pricing	27	[1]	22	[1]
Discounted Cash Flow | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.60%	[1]
Counterparty credit risk	2.00%	[2],[3]	1.00%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Maximum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.50%	[2],[3]	1.00%	[2],[3]
Credit risk	2.00%	[2],[3]	5.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	51	[3]	47	[3]
Estimated auction price for FTRs	945	[1]	1,851	[1]
Nodal basis	(1)	[3]	(1)	[3]
Discounted Cash Flow | Maximum | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	41	[1]	46	[1]
Discounted Cash Flow | Maximum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.60%	[1]
Counterparty credit risk	2.00%	[2],[3]	1.00%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Maximum | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.50%	[2],[3]	1.00%	[2],[3]
Credit risk	2.00%	[2],[3]	2.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	51	[3]	56	[3]
Estimated auction price for FTRs	945	[1]	1,851	[1]
Nodal basis	(1)	[3]	(1)	[3]
Discounted Cash Flow | Maximum | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	41	[1]	46	[1]
Discounted Cash Flow | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	2.00%	[2],[3]	5.00%	[2],[3]
Nodal basis	0	[1]	(1)	[1]
Average bid/ask consensus pricing	36	[1]	47	[1]
Discounted Cash Flow | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.44%	[1]
Counterparty credit risk	1.00%	[2],[3]	1.00%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Weighted Average | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.42%	[2],[3]	1.00%	[2],[3]
Credit risk	2.00%	[2],[3]	5.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	32	[3]	31	[3]
Estimated auction price for FTRs	305	[1]	178	[1]
Nodal basis	(2)	[3]	(3)	[3]
Discounted Cash Flow | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	36	[1]	44	[1]
Discounted Cash Flow | Weighted Average | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate			0.44%	[1]
Counterparty credit risk	1.00%	[2],[3]	1.00%	[2],[3]
Credit risk			2.00%	[2],[3]
Discounted Cash Flow | Weighted Average | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.42%	[2],[3]	1.00%	[2],[3]
Credit risk	2.00%	[2],[3]	2.00%	[2],[3]
Average bid/ask consensus peak and off-peak pricing	40	[3]	36	[3]
Estimated auction price for FTRs	305	[1]	178	[1]
Nodal basis	(2)	[3]	(2)	[3]
Discounted Cash Flow | Weighted Average | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	36	[1]	44	[1]
Discounted Cash Flow | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	2.00%	[2],[3]	5.00%	[2],[3]
Nodal basis	(2)	[1]	(3)	[1]
Average bid/ask consensus pricing	30	[1]	30	[1]
Option Model | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	10.00%	[1]	7.00%	[1]
Option Model | Minimum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	10.00%	[1]	7.00%	[1]
Option Model | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	35.00%	[1]	27.00%	[1]
Option Model | Maximum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	35.00%	[1]	27.00%	[1]
Option Model | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	16.00%	[1]	24.00%	[1]
Option Model | Weighted Average | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	16.00%	[1]	24.00%	[1]
Fundamental Energy Production Model | Minimum | Power
Fair Value Inputs [Abstract]
Escalation rate	3.00%	[1],[4]
Estimated future gas prices	4	[1]	4	[1]
Fundamental Energy Production Model | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	3.00%	[1],[4]
Estimated future gas prices	4	[1]	4	[1]
Fundamental Energy Production Model | Maximum | Power
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[1],[4]
Estimated future gas prices	5	[1]	8	[1]
Fundamental Energy Production Model | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[1],[4]
Estimated future gas prices	5	[1]	8	[1]
Fundamental Energy Production Model | Weighted Average | Power
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[1],[4]
Estimated future gas prices	5	[1]	6	[1]
Fundamental Energy Production Model | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	4.00%	[1],[4]
Estimated future gas prices	5	[1]	6	[1]
Contract Price Allocation | Minimum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[1]	5	[1]
Contract Price Allocation | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[1]	5	[1]
Contract Price Allocation | Maximum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[1]	7	[1]
Contract Price Allocation | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[1]	7	[1]
Contract Price Allocation | Weighted Average | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[1]	6	[1]
Contract Price Allocation | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[1]	6	[1]
Derivative liabilities | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ (3)	[5]	$ (3)	[5]
Derivative liabilities | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(110)	[5],[6]	(114)	[5],[6]
Derivative liabilities | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(6)	[5]	(2)	[5]
Derivative liabilities | Union Electric Company | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(3)	[5]	(3)	[5]
Derivative liabilities | Union Electric Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(2)	[5],[6]	(3)	[5],[6]
Derivative liabilities | Union Electric Company | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(6)	[5]	(2)	[5]
Derivative liabilities | Ameren Illinois Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(108)	[5],[6]	(111)	[5],[6]
Derivative assets | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	8	[5]	8	[5]
Derivative assets | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	21	[5],[6]	14	[5],[6]
Derivative assets | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[5]		[5]
Derivative assets | Union Electric Company | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	8	[5]	8	[5]
Derivative assets | Union Electric Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	21	[5],[6]	14	[5],[6]
Derivative assets | Union Electric Company | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[5]		[5]
Derivative assets | Ameren Illinois Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		[5],[6]		[5],[6]

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

[2] Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren, Ameren Missouri and Ameren Illinois credit risk is applied only 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] Escalation rate applies to power prices 2026 and beyond.

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

[6] 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.