Commitments And Contingencies (Other Obligations) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Long-term Purchase Commitment [Line Items]
2014	$ 457	[1]
2015	1,265	[1]
2016	1,040	[1]
2017	941	[1]
2018	324	[1]
Thereafter	1,480	[1]
Total	5,507	[1]
Renewable Energy Credits Agreements, Term	20 years
Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	294
2015	864
2016	813
2017	805
2018	233
Thereafter	716
Total	3,725
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014	144
2015	383
2016	214
2017	136
2018	91
Thereafter	764
Total	1,732
Investment in Energy Efficiency Programs | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	17
2015	64
Coal
Long-term Purchase Commitment [Line Items]
2014	151	[1]
2015	635	[1]
2016	659	[1]
2017	682	[1]
2018	111	[1]
Thereafter	114	[1]
Total	2,352	[1]
Coal | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	151
2015	635
2016	659
2017	682
2018	111
Thereafter	114
Total	2,352
Coal | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014
2015
2016
2017
2018
Thereafter
Total
Natural Gas
Long-term Purchase Commitment [Line Items]
2014	93	[1],[2]
2015	225	[1],[2]
2016	127	[1],[2]
2017	80	[1],[2]
2018	41	[1],[2]
Thereafter	101	[1],[2]
Total	667	[1],[2]
Natural Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	16	[2]
2015	39	[2]
2016	21	[2]
2017	13	[2]
2018	8	[2]
Thereafter	29	[2]
Total	126	[2]
Natural Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014	77	[2]
2015	186	[2]
2016	106	[2]
2017	67	[2]
2018	33	[2]
Thereafter	72	[2]
Total	541	[2]
Nuclear Fuel
Long-term Purchase Commitment [Line Items]
2014	62	[1]
2015	56	[1]
2016	69	[1]
2017	59	[1]
2018	61	[1]
Thereafter	179	[1]
Total	486	[1]
Nuclear Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	62
2015	56
2016	69
2017	59
2018	61
Thereafter	179
Total	486
Nuclear Fuel | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014
2015
2016
2017
2018
Thereafter
Total
Purchased Power
Long-term Purchase Commitment [Line Items]
2014	62	[1],[3]
2015	190	[1],[3]
2016	105	[1],[3]
2017	66	[1],[3]
2018	55	[1],[3]
Thereafter	645	[1],[3]
Total	1,123	[1],[3]
Purchased Power | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	4
2015	21
2016	21
2017	21
2018	21
Thereafter	120
Total	208
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014	58	[3]
2015	169	[3]
2016	84	[3]
2017	45	[3]
2018	34	[3]
Thereafter	525	[3]
Total	915	[3]
Methane Gas
Long-term Purchase Commitment [Line Items]
2014	1	[1]
2015	3	[1]
2016	4	[1]
2017	4	[1]
2018	5	[1]
Thereafter	91	[1]
Total	108	[1]
Methane Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	1
2015	3
2016	4
2017	4
2018	5
Thereafter	91
Total	108
Methane Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014
2015
2016
2017
2018
Thereafter
Total
Other Commitments
Long-term Purchase Commitment [Line Items]
2014	88	[1]
2015	156	[1]
2016	76	[1]
2017	50	[1]
2018	51	[1]
Thereafter	350	[1]
Total	771	[1]
Other Commitments | Union Electric Company
Long-term Purchase Commitment [Line Items]
2014	60
2015	110
2016	39
2017	26
2018	27
Thereafter	183
Total	445
Other Commitments | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2014	9
2015	28
2016	24
2017	24
2018	24
Thereafter	167
Total	$ 276

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] Includes amounts for generation and for distribution.

[3] The purchased power amounts for Ameren and Ameren Illinois include twenty-year agreements for renewable energy credits that were entered into in December 2010 with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits.