Commitments And Contingencies (Schedule of Estimated Purchase Commitments) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Long-term Purchase Commitment [Line Items]
2013	$ 981	[1]
2014	1,440	[1]
2015	1,130	[1]
2016	966	[1]
2017	893	[1]
Thereafter	1,405	[1]
Total	6,815	[1]
Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	719
2014	884
2015	841
2016	825
2017	808
Thereafter	859
Total	4,936
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013	250
2014	547
2015	284
2016	141
2017	85
Thereafter	546
Total	1,853
Investment in Energy Efficiency Programs | Union Electric Company
Long-term Purchase Commitment [Line Items]
Other Commitment	147
Other Commitments, Term	3 years
Coal
Long-term Purchase Commitment [Line Items]
2013	605	[1]
2014	617	[1]
2015	640	[1]
2016	665	[1]
2017	683	[1]
Thereafter	245	[1]
Total	3,455	[1]
Coal | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	605
2014	617
2015	640
2016	665
2017	683
Thereafter	245
Total	3,455
Coal | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013
2014
2015
2016
2017
Thereafter
Total
Natural Gas
Long-term Purchase Commitment [Line Items]
2013	109	[1]
2014	292	[1]
2015	155	[1]
2016	85	[1]
2017	47	[1]
Thereafter	100	[1]
Total	788	[1]
Natural Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	27
2014	50
2015	30
2016	16
2017	11
Thereafter	28
Total	162
Natural Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013	82
2014	242
2015	125
2016	69
2017	36
Thereafter	72
Total	626
Nuclear Fuel
Long-term Purchase Commitment [Line Items]
2013	17	[1]
2014	68	[1]
2015	63	[1]
2016	81	[1]
2017	58	[1]
Thereafter	216	[1]
Total	503	[1]
Nuclear Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	17
2014	68
2015	63
2016	81
2017	58
Thereafter	216
Total	503
Nuclear Fuel | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013
2014
2015
2016
2017
Thereafter
Total
Purchased Power
Long-term Purchase Commitment [Line Items]
2013	162	[1],[2]
2014	302	[1],[2]
2015	154	[1],[2]
2016	67	[1],[2]
2017	44	[1],[2]
Thereafter	501	[1],[2]
Total	1,230	[1],[2]
Purchased Power | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	3	[2]
2014	19	[2]
2015	19	[2]
2016	19	[2]
2017	19	[2]
Thereafter	129	[2]
Total	208	[2]
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013	159	[2]
2014	283	[2]
2015	135	[2]
2016	48	[2]
2017	25	[2]
Thereafter	372	[2]
Total	1,022	[2]
Renewable Energy Credits Agreements, Term	20 years
Methane Gas
Long-term Purchase Commitment [Line Items]
2013	1	[1]
2014	3	[1]
2015	4	[1]
2016	4	[1]
2017	5	[1]
Thereafter	97	[1]
Total	114	[1]
Methane Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	1
2014	3
2015	4
2016	4
2017	5
Thereafter	97
Total	114
Methane Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013
2014
2015
2016
2017
Thereafter
Total
Other Purchased Fuel
Long-term Purchase Commitment [Line Items]
2013	87	[1]
2014	158	[1]
2015	114	[1]
2016	64	[1]
2017	56	[1]
Thereafter	246	[1]
Total	725	[1]
Other Purchased Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2013	66
2014	127
2015	85
2016	40
2017	32
Thereafter	144
Total	494
Other Purchased Fuel | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2013	9
2014	22
2015	24
2016	24
2017	24
Thereafter	102
Total	$ 205

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

[2] The purchased power amounts for Ameren and Ameren Illinois includes 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.