Commitments And Contingencies (Environmental Matters-Acts) (Detail) (USD $) In Millions, unless otherwise specified			6 Months Ended	0 Months Ended						6 Months Ended									12 Months Ended															6 Months Ended
	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 CAIR [Member] State	Dec. 21, 2011 MATS [Member]	Jun. 30, 2012 Estimated Capital Costs 2012 [Member]	Jun. 30, 2012 Estimated Capital Costs 2013 To 2016 [Member] Minimum [Member]	Jun. 30, 2012 Estimated Capital Costs 2013 To 2016 [Member] Maximum [Member]	Jun. 30, 2012 Estimated Capital Costs 2017 To 2021 [Member] Minimum [Member]	Jun. 30, 2012 Estimated Capital Costs 2017 To 2021 [Member] Maximum [Member]	Jun. 30, 2012 Genco [Member]	Jun. 30, 2012 Genco [Member] Minimum [Member]	Jun. 30, 2012 Genco [Member] Maximum [Member]	Jun. 30, 2012 Genco [Member] Estimated Capital Costs 2012 [Member]	Dec. 30, 2013 Genco [Member] Estimated Capital Costs 2013 To 2016 [Member]	Jun. 30, 2012 Genco [Member] Estimated Capital Costs 2013 To 2016 [Member] Minimum [Member]	Jun. 30, 2012 Genco [Member] Estimated Capital Costs 2013 To 2016 [Member] Maximum [Member]	Jun. 30, 2012 Genco [Member] Estimated Capital Costs 2017 To 2021 [Member] Minimum [Member]	Jun. 30, 2012 Genco [Member] Estimated Capital Costs 2017 To 2021 [Member] Maximum [Member]	Dec. 31, 2010 Ameren Missouri [Member]	Jun. 30, 2012 Ameren Missouri [Member] Minimum [Member]		Jun. 30, 2012 Ameren Missouri [Member] Maximum [Member]		Jun. 30, 2012 Ameren Missouri [Member] Estimated Capital Costs 2012 [Member]		Jun. 30, 2012 Ameren Missouri [Member] Estimated Capital Costs 2013 To 2016 [Member] Minimum [Member]		Jun. 30, 2012 Ameren Missouri [Member] Estimated Capital Costs 2013 To 2016 [Member] Maximum [Member]		Jun. 30, 2012 Ameren Missouri [Member] Estimated Capital Costs 2017 To 2021 [Member] Minimum [Member]		Jun. 30, 2012 Ameren Missouri [Member] Estimated Capital Costs 2017 To 2021 [Member] Maximum [Member]		Jun. 30, 2012 Genco And AERG [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Minimum [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Maximum [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Estimated Capital Costs 2012 [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Estimated Capital Costs 2013 To 2016 [Member] Minimum [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Estimated Capital Costs 2013 To 2016 [Member] Maximum [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Estimated Capital Costs 2017 To 2021 [Member] Minimum [Member]	Jun. 30, 2012 Ameren Energy Resources Generating Company [Member] Estimated Capital Costs 2017 To 2021 [Member] Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 1,825	$ 2,185			$ 210	$ 445	$ 550	$ 1,170	$ 1,425		$ 495	$ 570	$ 150	$ 20	$ 100	$ 125	$ 245	$ 295		$ 1,225	[1]	$ 1,485	[1]	$ 55	[1]	$ 325	[1]	$ 400	[1]	$ 845	[1]	$ 1,030	[1]		$ 105	$ 130	$ 5	$ 20	$ 25	$ 80	$ 100
Number of states participating in the cap-and-trade program			28
Percent of top performing facilities				12.00%
Number of scrubbers installed																			2															3
Number of additional scrubbers										2

[1] Ameren Missouri's expenditures are expected to be recoverable from ratepayers.