Commitments And Contingencies (Environmental Matters) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Loss Contingencies [Line Items]
EPA Clean Power Plan CO2 Proposed Reductions	30.00%
Estimated Environmental Expenditures to Comply with Clean Power Plan	$ 2,000,000,000
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350,000,000
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400,000,000
Former Coal Tar Distillery | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	2,000,000
Range of possible loss maximum	5,000,000
Accrual for environmental loss contingencies	2,000,000
Former Coal Ash Landfill | Ameren Illinois Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	500,000
Range of possible loss maximum	6,000,000
Accrual for environmental loss contingencies	500,000
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	245,000,000
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites	44
Range of possible loss, minimum	245,000,000
Range of possible loss maximum	314,000,000
Accrual for environmental loss contingencies	245,000,000
Other Environmental | Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	700,000
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	1,000,000
Range of possible loss maximum	2,500,000
Accrual for environmental loss contingencies	1,000,000
Substation in St Charles, Missouri | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	2,300,000
Range of possible loss maximum	4,500,000
Accrual for environmental loss contingencies	$ 2,300,000