Commitments And Contingencies (Environmental Matters) (Details) (USD $)	Sep. 30, 2011		Dec. 31, 2010
Property, plant and equipment, net	$ 17,873,000,000		$ 17,853,000,000
Minimum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,900,000,000
Minimum [Member] | Estimated Capital Costs 2012 - 2015 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	790,000,000
Minimum [Member] | Estimated Capital Costs 2016 - 2020 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	970,000,000
Maximum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	2,295,000,000
Maximum [Member] | Estimated Capital Costs 2012 - 2015 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	970,000,000
Maximum [Member] | Estimated Capital Costs 2016 - 2020 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,185,000,000
Ameren Missouri [Member] | Manufactured Gas Plant [Member]
Number of remediation sites	10
Ameren Illinois Company [Member] | Manufactured Gas Plant [Member]
Number of remediation sites	44
Manufactured Gas Plant [Member]
Loss contingency range of possible loss minimum	112,000,000
Loss contingency range of possible loss maximum	191,000,000
Accrual for environmental loss contingencies	112,000,000	[1]
Ameren Illinois Company [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 Company [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	300,000
Loss contingency range of possible loss maximum	10,000,000
Accrual for environmental loss contingencies	300,000
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	140,000,000
Ameren Illinois Company [Member]
Property, plant and equipment, net	$ 4,699,000,000		$ 4,576,000,000

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