Commitments and Contingencies (Environmental Matters) (Details) (USD $)	Jun. 30, 2011 properties		Dec. 31, 2010
MACT standard emission limits based on existing coal and oil-fired generating units, percentage	12.00%
Reduction in mercury emissions included in the proposed federal MACT standard	91.00%
Number of states included in the CAIR regulations	28
Number of states included in CSAPR	23
Expected percentage reduction in SO2 emissions by 2014 included in CSAPR	73.00%
Expected percentage reduction in NOx emissions by 2014 included in the proposed transport rule	54.00%
Threshold amount of greenhouse emissions in tons that will require operating permit under title V operating permit program of the Clean Air Act	75,000
Threshold, in millions of gallons of water per day, for power plants to be regulated under the EPA's proposed Clean Water Act rules	2
Threshold, in percentage, of water used for cooling at a power plant to be regulated under the EPA's proposed Clean Water Act rules	25.00%
Level of intake velocity (feet per second) included in the EPA's proposed Clean Water Act rules	0.5
Property, plant and equipment, net	$ 17,945,000,000		$ 17,853,000,000
Hutsonville [Member]
Property, plant and equipment, net	26,000,000
Meredosia [Member]
Property, plant and equipment, net	1,000,000
Unit One, E.D. Edwards Energy Center [Member]
Property, plant and equipment, net	18,000,000
Minimum [Member] | Estimated Capital Costs 2016 - 2020 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	975,000,000
Minimum [Member] | Estimated Capital Costs 2011 Through 2020[Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,990,000,000
Maximum [Member] | Estimated Capital Costs 2012 - 2015 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,035,000,000
Maximum [Member] | Estimated Capital Costs 2016 - 2020 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,195,000,000
Maximum [Member] | Estimated Capital Costs 2011 Through 2020[Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	2,405,000,000
Ameren Missouri [Member] | Manufactured Gas Plant [Member]
Number of remediation sites	10
Ameren Illinois [Member] | Manufactured Gas Plant [Member]
Number of remediation sites	44
Manufactured Gas Plant [Member]
Loss contingency range of possible loss minimum	121,000,000
Loss contingency range of possible loss maximum	199,000,000
Accrual for environmental loss contingencies	121,000,000	[1]
Manufactured Gas Plant [Member] | Iowa [Member]
Number of remediation sites	1
Ameren Illinois [Member] | Former Coal Ash Landfill [Member]
Loss contingency range of possible loss minimum	500,000
Loss contingency range of possible loss maximum	6,000,000
Accrual for environmental loss contingencies	500,000
Ameren Illinois [Member] | Other Environmental [Member]
Accrual for environmental loss contingencies	800,000
Ameren Missouri [Member] | Former Coal Tar Distillery [Member]
Loss contingency range of possible loss minimum	2,000,000
Loss contingency range of possible loss maximum	5,000,000
Accrual for environmental loss contingencies	2,000,000
Ameren Missouri [Member] | Sauget Area 2 [Member]
Loss contingency range of possible loss minimum	400,000
Loss contingency range of possible loss maximum	10,000,000
Accrual for environmental loss contingencies	400,000
Estimated Capital Costs [Member]
Reduction in capital expenditure estimate for environmental compliance	1,100,000,000
Estimated Capital Costs 2011 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175,000,000
Ameren Illinois [Member]
Property, plant and equipment, net	$ 4,657,000,000		$ 4,576,000,000

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