Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2016 USD ($) site
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 600.0
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 700.0
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers	2
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 224.0
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 224.0
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	224.0
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	313.0
Former Coal Ash Landfill \| Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.5
Former Coal Ash Landfill \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	0.5
Former Coal Ash Landfill \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	6.0
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]
Accrual for environmental loss contingencies	2.0
Former Coal Tar Distillery \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	2.0
Former Coal Tar Distillery \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	5.0
Sauget Area Two \| Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	1.0
Sauget Area Two \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	1.0
Sauget Area Two \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	2.5
Substation in St Charles, Missouri \| Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 0.6

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

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- Definition

Number Of Remediation Sites

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- Definition

Number of Energy Center Scrubbers to achieve compliance with regulations

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- Definition

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
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
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-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

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-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

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