Commitments And Contingencies (Commitments) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Ameren [Member]
Commitments [Line Items]
2012	$ 966	[1]
2013	1,793	[1]
2014	1,521	[1]
2015	1,151	[1]
2016	978	[1]
Thereafter	2,553	[1]
Total	8,962	[1]
Ameren Missouri [Member]
Commitments [Line Items]
2012	703
2013	777
2014	851
2015	784
2016	809
Thereafter	1,706
Total	5,630
Ameren Illinois Company [Member]
Commitments [Line Items]
2012	95
2013	710
2014	508
2015	277
2016	128
Thereafter	790
Total	2,508
Genco [Member]
Commitments [Line Items]
2012	117
2013	215
2014	110
2015	59
2016	10
Thereafter	11
Total	522
Coal [Member] | Ameren [Member]
Commitments [Line Items]
2012	727	[1]
2013	871	[1]
2014	774	[1]
2015	702	[1]
2016	685	[1]
Thereafter	978	[1]
Total	4,737	[1]
Coal [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	614
2013	609
2014	625
2015	614
2016	644
Thereafter	921
Total	4,027
Coal [Member] | Genco [Member]
Commitments [Line Items]
2012	86
2013	177
2014	100
2015	57
2016	10
Thereafter	11
Total	441
Natural Gas [Member] | Ameren [Member]
Commitments [Line Items]
2012	82	[1]
2013	372	[1]
2014	243	[1]
2015	134	[1]
2016	55	[1]
Thereafter	130	[1]
Total	1,016	[1]
Natural Gas [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	16
2013	57
2014	43
2015	24
2016	10
Thereafter	36
Total	186
Natural Gas [Member] | Ameren Illinois Company [Member]
Commitments [Line Items]
2012	66
2013	287
2014	197
2015	108
2016	45
Thereafter	94
Total	797
Natural Gas [Member] | Genco [Member]
Commitments [Line Items]
2013	28
2014	3
2015	2
Total	33
Nuclear Fuel [Member] | Ameren [Member]
Commitments [Line Items]
2012	22	[1]
2013	35	[1]
2014	93	[1]
2015	86	[1]
2016	104	[1]
Thereafter	346	[1]
Total	686	[1]
Nuclear Fuel [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	22
2013	35
2014	93
2015	86
2016	104
Thereafter	346
Total	686
Purchased Power [Member] | Ameren [Member]
Commitments [Line Items]
2012	24	[1],[2]
2013	420	[1],[2]
2014	308	[1],[2]
2015	164	[1],[2]
2016	78	[1],[2]
Thereafter	749	[1],[2]
Total	1,743	[1],[2]
Purchased Power [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	3	[2]
2013	19	[2]
2014	19	[2]
2015	19	[2]
2016	19	[2]
Thereafter	155	[2]
Total	234	[2]
Purchased Power [Member] | Ameren Illinois Company [Member]
Commitments [Line Items]
2012	21	[2]
2013	401	[2]
2014	289	[2]
2015	145	[2]
2016	59	[2]
Thereafter	594	[2]
Total	1,509	[2]
Methane Gas [Member] | Ameren [Member]
Commitments [Line Items]
2012	1	[1]
2013	3	[1]
2014	3	[1]
2015	4	[1]
2016	4	[1]
Thereafter	104	[1]
Total	119	[1]
Methane Gas [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	1
2013	3
2014	3
2015	4
2016	4
Thereafter	104
Total	119
Others [Member] | Ameren [Member]
Commitments [Line Items]
2012	110	[1]
2013	92	[1]
2014	100	[1]
2015	61	[1]
2016	52	[1]
Thereafter	246	[1]
Total	661	[1]
Others [Member] | Ameren Missouri [Member]
Commitments [Line Items]
2012	47
2013	54
2014	68
2015	37
2016	28
Thereafter	144
Total	378
Others [Member] | Ameren Illinois Company [Member]
Commitments [Line Items]
2012	8
2013	22
2014	22
2015	24
2016	24
Thereafter	102
Total	202
Others [Member] | Genco [Member]
Commitments [Line Items]
2012	31
2013	10
2014	7
Total	$ 48

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

[2] The purchased power amounts for Ameren and Ameren Illinois include a 20-year agreement for renewable energy credits that was 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.