Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Sep. 30, 2015 USD ($) site
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 350.0
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400.0
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 237.0
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Range of possible loss, minimum	$ 237.0
Range of possible loss maximum	309.0
Accrual for environmental loss contingencies	237.0
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
Other Environmental \| Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.7
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
Sauget Area Two \| Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	1.0
Range of possible loss maximum	2.5
Accrual for environmental loss contingencies	1.0
Substation in St Charles, Missouri \| Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 0.8

Estimated capital costs to comply with existing and known federal and state air emissions regulations

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Number Of Remediation Sites

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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