Commitments And Contingencies (Environmental Matters) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 state
Number of states participating in the cap-and-trade program	28
Percent of top performing facilities	12.00%
Estimated Capital Costs 2012 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	140
Estimated Capital Costs 2012 [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	105	[1]
Estimated Capital Costs 2012 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	30
Estimated Capital Costs 2012 [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	5
Newton Energy Center Scrubbers [Member] | Estimated Capital Costs 2013 to 2017 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	20
Manufactured Gas Plant [Member]
Loss contingency range of possible loss, minimum	257.0
Loss contingency range of possible loss, maximum	339.0
Accrual for environmental loss contingencies	257.0	[2]
Manufactured Gas Plant [Member] | Union Electric Company [Member]
Loss contingency range of possible loss, minimum	5.0
Loss contingency range of possible loss, maximum	6.0
Accrual for environmental loss contingencies	5.0	[2]
Manufactured Gas Plant [Member] | Ameren Illinois Company [Member]
Number of remediation sites	44
Loss contingency range of possible loss, minimum	252.0
Loss contingency range of possible loss, maximum	333.0
Accrual for environmental loss contingencies	252.0	[2]
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	6.0
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] | Union Electric Company [Member]
Loss contingency range of possible loss, minimum	2.0
Loss contingency range of possible loss, maximum	5.0
Accrual for environmental loss contingencies	2.0
Sauget Area 2 [Member] | Union Electric Company [Member]
Loss contingency range of possible loss, minimum	0.3
Loss contingency range of possible loss, maximum	10.0
Accrual for environmental loss contingencies	0.3
Substation in St Charles, Missouri [Member] | Union Electric Company [Member]
Loss contingency range of possible loss, minimum	1.5
Loss contingency range of possible loss, maximum	2.3
Accrual for environmental loss contingencies	1.5
Minimum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,510
Minimum [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,115	[1]
Minimum [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350
Minimum [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	45
Minimum [Member] | Estimated Capital Costs 2014 to 2017 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	335
Minimum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	215	[1]
Minimum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	100
Minimum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	20
Minimum [Member] | Estimated Capital Costs 2018 to 2022 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,035
Minimum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	795	[1]
Minimum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	220
Minimum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	20
Maximum [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,820
Maximum [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,340	[1]
Maximum [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425
Maximum [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	55
Maximum [Member] | Estimated Capital Costs 2014 to 2017 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	410
Maximum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	260	[1]
Maximum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	125
Maximum [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	25
Maximum [Member] | Estimated Capital Costs 2018 to 2022 [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,270
Maximum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Union Electric Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	975	[1]
Maximum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Ameren Energy Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	270
Maximum [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Ameren Energy Resources Generating Company [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	25

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

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