Commitments And Contingencies (Environmental Matters) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Number of states participating in the cap-and-trade program	28
Percent of top performing facilities	12.00%
Property, plant and equipment, net	$ 18,127	[1],[2]	$ 17,853	[1],[2]
Ameren Illinois Company [Member]
Property, plant and equipment, net	4,770		4,576
Ameren Energy Generating Company [Member]
Property, plant and equipment, net	2,231		2,248
Estimated Capital Costs 2012 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	210
Estimated Capital Costs 2012 [Member] | Ameren Energy Generating Company [Member]
Reduction in capital expenditure estimate for environmental compliance	150
Estimated Capital Costs 2013 - 2016 [Member] | Ameren Energy Generating Company [Member]
Reduction in capital expenditure estimate for environmental compliance	20
Manufactured Gas Plant [Member]
Loss contingency range of possible loss minimum	107.0
Loss contingency range of possible loss maximum	183
Accrual for environmental loss contingencies	107.0	[3]
Manufactured Gas Plant [Member] | Ameren Illinois Company [Member]
Number of remediation sites	44
Manufactured Gas Plant [Member] | Ameren Missouri [Member]
Number of remediation sites	10
Former Coal Ash Landfill [Member] | Ameren Illinois Company [Member]
Loss contingency range of possible loss minimum	0.5
Loss contingency range of possible loss maximum	6
Accrual for environmental loss contingencies	0.5
Other Environmental [Member] | Ameren Illinois Company [Member]
Accrual for environmental loss contingencies	0.8
Former Coal Tar Distillery [Member] | Ameren Missouri [Member]
Loss contingency range of possible loss minimum	2.0
Loss contingency range of possible loss maximum	5
Accrual for environmental loss contingencies	2.0
Sauget Area 2 [Member] | Ameren Missouri [Member]
Loss contingency range of possible loss minimum	0.3
Loss contingency range of possible loss maximum	10
Accrual for environmental loss contingencies	0.3
Minimum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,825
Minimum [Member] | Estimated Capital Costs 2013 - 2016 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	445
Minimum [Member] | Estimated Capital Costs 2017 - 2021 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,170
Maximum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	2,185
Maximum [Member] | Estimated Capital Costs 2013 - 2016 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	550
Maximum [Member] | Estimated Capital Costs 2017 - 2021 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 1,425

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

[2] Amounts in Ameren and Ameren Missouri include two electric generation CTs under two separate capital lease agreements. The gross asset value of those agreements was $229 million and $228 million at December 31, 2011 and 2010, respectively. The total accumulated depreciation associated with the two CTs was $52 million and $46 million at December 31, 2011 and 2010, respectively.

[3] Recorded liability represents the estimated minimum probable obligations, as no other amount within the range provided a better estimate.