Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	$ 1,891	[1]
2014	1,596	[1]
2015	1,212	[1]
2016	1,025	[1]
2017	923	[1]
Thereafter	1,691	[1]
Total	8,338	[1]
Coal [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	908	[1]
2014	774	[1]
2015	702	[1]
2016	732	[1]
2017	701	[1]
Thereafter	277	[1]
Total	4,094	[1]
Natural Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	349	[1]
2014	254	[1]
2015	138	[1]
2016	54	[1]
2017	34	[1]
Thereafter	105	[1]
Total	934	[1]
Nuclear Fuel [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	36	[1]
2014	89	[1]
2015	87	[1]
2016	95	[1]
2017	78	[1]
Thereafter	277	[1]
Total	662	[1]
Purchased Power [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	421	[1],[2]
2014	309	[1],[2]
2015	164	[1],[2]
2016	78	[1],[2]
2017	55	[1],[2]
Thereafter	687	[1],[2]
Total	1,714	[1],[2]
Renewable Energy Credits Agreements, Term	20 years
Methane Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	3	[1]
2014	3	[1]
2015	4	[1]
2016	4	[1]
2017	5	[1]
Thereafter	99	[1]
Total	118	[1]
Other [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	174	[1]
2014	167	[1]
2015	117	[1]
2016	62	[1]
2017	50	[1]
Thereafter	246	[1]
Total	816	[1]
Union Electric Company [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	841
2014	902
2015	836
2016	810
2017	809
Thereafter	923
Total	5,121
Union Electric Company [Member] | Coal [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	620
2014	625
2015	614
2016	644
2017	676
Thereafter	245
Total	3,424
Union Electric Company [Member] | Natural Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	57
2014	43
2015	25
2016	10
2017	5
Thereafter	28
Total	168
Union Electric Company [Member] | Nuclear Fuel [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	36
2014	89
2015	87
2016	95
2017	78
Thereafter	277
Total	662
Union Electric Company [Member] | Purchased Power [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	19	[2]
2014	19	[2]
2015	19	[2]
2016	19	[2]
2017	19	[2]
Thereafter	130	[2]
Total	225	[2]
Union Electric Company [Member] | Methane Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	3
2014	3
2015	4
2016	4
2017	5
Thereafter	99
Total	118
Union Electric Company [Member] | Other [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	106
2014	123
2015	87
2016	38
2017	26
Thereafter	144
Total	524
Ameren Illinois Company [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	695
2014	517
2015	279
2016	127
2017	89
Thereafter	739
Total	2,446
Long-term Purchase Commitment, Cubic Feet To Be Purchased	15,500
Gas Transportation Agreements, Term	10 years
Ameren Illinois Company [Member] | Coal [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013
2014
2015
2016
2017
Thereafter
Total
Ameren Illinois Company [Member] | Natural Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	270
2014	206
2015	110
2016	44
2017	29
Thereafter	78
Total	737
Ameren Illinois Company [Member] | Nuclear Fuel [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013
2014
2015
2016
2017
Thereafter
Total
Ameren Illinois Company [Member] | Purchased Power [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	401	[2]
2014	289	[2]
2015	145	[2]
2016	59	[2]
2017	36	[2]
Thereafter	559	[2]
Total	1,489	[2]
Ameren Illinois Company [Member] | Methane Gas [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013
2014
2015
2016
2017
Thereafter
Total
Ameren Illinois Company [Member] | Other [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2013	24
2014	22
2015	24
2016	24
2017	24
Thereafter	102
Total	220
Investment in Energy Efficiency Programs [Member] | Union Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Other commitment	$ 147
Other commitments, term	3 years

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

[2] The purchased power amounts for Ameren and Ameren Illinois includes 20-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.