Commitments And Contingencies (Environmental Matters) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 State
Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	395	[1]
Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	480	[1]
Estimated Capital Costs 2012 [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	35	[1]
Estimated Capital Costs 2014 to 2017 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	120	[1]
Estimated Capital Costs 2014 to 2017 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	150	[1]
Estimated Capital Costs 2018 to 2022 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	240	[1]
Estimated Capital Costs 2018 to 2022 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	295	[1]
Ameren Energy Generating Company [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	350	[2]
Ameren Energy Generating Company [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	425	[2]
Ameren Energy Generating Company [Member] | Estimated Capital Costs 2012 [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	30	[2]
Ameren Energy Generating Company [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	100	[2]
Ameren Energy Generating Company [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	125	[2]
Ameren Energy Generating Company [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	220	[2]
Ameren Energy Generating Company [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	270	[2]
Union Electric Company [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	1,115	[3]
Union Electric Company [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	1,340	[3]
Union Electric Company [Member] | Estimated Capital Costs 2012 [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	105	[3]
Union Electric Company [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	215	[3]
Union Electric Company [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	260	[3]
Union Electric Company [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	795	[3]
Union Electric Company [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	975	[3]
AERG [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	45
AERG [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	55
AERG [Member] | Estimated Capital Costs 2012 [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	5
AERG [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	20
AERG [Member] | Estimated Capital Costs 2014 to 2017 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	25
AERG [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	20
AERG [Member] | Estimated Capital Costs 2018 to 2022 [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	25
CAIR [Member]
Loss Contingencies [Line Items]
Number of states participating in the cap-and-trade program	28
MATS [Member]
Loss Contingencies [Line Items]
Percent of top performing facilities	12.00%
Former Coal Tar Distillery [Member] | Union Electric Company [Member]
Loss Contingencies [Line Items]
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
Former Coal Ash Landfill [Member] | Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
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
Newton Energy Center Scrubbers [Member] | Ameren Energy Generating Company [Member] | Newton Energy Center Estimated Capital Costs 2013 to 2017 [Member]
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	20
Manufactured Gas Plant [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss minimum	259.0
Loss contingency range of possible loss maximum	342.0
Accrual for environmental loss contingencies	259.0	[4]
Manufactured Gas Plant [Member] | Union Electric Company [Member]
Loss Contingencies [Line Items]
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	[4]
Manufactured Gas Plant [Member] | Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
Number of remediation sites	44
Loss contingency range of possible loss minimum	254.0
Loss contingency range of possible loss maximum	336.0
Accrual for environmental loss contingencies	254.0	[4]
Other Environmental [Member] | Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.8
Sauget Area Two [Member] | Union Electric Company [Member]
Loss Contingencies [Line Items]
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 Contingencies [Line Items]
Loss contingency range of possible loss minimum	1.7
Loss contingency range of possible loss maximum	4.5
Accrual for environmental loss contingencies	1.7

[1] Assumes the Merchant Generation facilities are owned by Ameren.

[2] ncludes estimated costs of approximately $20 million annually, excluding capitalized interest, from 2013 through 2017 for construction of two Newton energy center scrubbers.

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

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