Commitments And Contingencies (Environmental Matters) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013 State
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	$ 395	[1]
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	480	[1]
Estimated Capital Costs 2013
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 | Minimum
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 | Maximum
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 | Minimum
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 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	295	[1]
Ameren Energy Generating Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	350	[2]
Ameren Energy Generating Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	425	[2]
Ameren Energy Generating Company | Estimated Capital Costs 2013
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	30	[2]
Ameren Energy Generating Company | Estimated Capital Costs 2014 to 2017 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	100	[2]
Ameren Energy Generating Company | Estimated Capital Costs 2014 to 2017 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	125	[2]
Ameren Energy Generating Company | Estimated Capital Costs 2018 to 2022 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	220	[2]
Ameren Energy Generating Company | Estimated Capital Costs 2018 to 2022 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	270	[2]
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	1,115	[3]
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	1,340	[3]
Union Electric Company | Estimated Capital Costs 2013
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	105	[3]
Union Electric Company | Estimated Capital Costs 2014 to 2017 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	215	[3]
Union Electric Company | Estimated Capital Costs 2014 to 2017 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	260	[3]
Union Electric Company | Estimated Capital Costs 2018 to 2022 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	795	[3]
Union Electric Company | Estimated Capital Costs 2018 to 2022 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	975	[3]
AERG | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	45
AERG | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	55
AERG | Estimated Capital Costs 2013
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	5
AERG | Estimated Capital Costs 2014 to 2017 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	20
AERG | Estimated Capital Costs 2014 to 2017 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	25
AERG | Estimated Capital Costs 2018 to 2022 | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	20
AERG | Estimated Capital Costs 2018 to 2022 | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state environmental regulations	25
CAIR
Loss Contingencies [Line Items]
Number of states participating in the cap-and-trade program	28
MATS
Loss Contingencies [Line Items]
Percent of top performing facilities	12.00%
Former Coal Tar Distillery | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	2.0
Range of possible loss maximum	5.0
Accrual for environmental loss contingencies	2.0
Former Coal Ash Landfill | Ameren Illinois Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	0.5
Range of possible loss maximum	6.0
Accrual for environmental loss contingencies	0.5
Newton Energy Center Scrubbers | Ameren Energy Generating Company | Newton Energy Center Estimated Capital Costs 2013 to 2017
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	20
Manufactured Gas Plant
Loss Contingencies [Line Items]
Range of possible loss, minimum	251.0
Range of possible loss maximum	337.0
Accrual for environmental loss contingencies	251.0	[4]
Manufactured Gas Plant | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	5.0
Range of possible loss maximum	6.0
Accrual for environmental loss contingencies	5.0	[4]
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites	44
Range of possible loss, minimum	246.0
Range of possible loss maximum	331.0
Accrual for environmental loss contingencies	246.0	[4]
Other Environmental | Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.8
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	0.3
Range of possible loss maximum	10.0
Accrual for environmental loss contingencies	0.3
Substation in St Charles, Missouri | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	1.6
Range of possible loss maximum	4.5
Accrual for environmental loss contingencies	$ 1.6

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

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

[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 was a better estimate.