Commitments And Contingencies (Environmental Matters) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 275
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350
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
Manufactured Gas Plant
Loss Contingencies [Line Items]
Range of possible loss, minimum	269.0
Range of possible loss maximum	345.0
Accrual for environmental loss contingencies	269.0	[1]
Manufactured Gas Plant | Union Electric Company
Loss Contingencies [Line Items]
Number of remediation sites	1
Range of possible loss, minimum	3.0
Range of possible loss maximum	4.0
Accrual for environmental loss contingencies	3.0	[1]
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites	44
Range of possible loss, minimum	266.0
Range of possible loss maximum	341.0
Accrual for environmental loss contingencies	266.0	[1]
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	1.0
Range of possible loss maximum	2.5
Loss Contingency, Loss in Period	0.7
Accrual for environmental loss contingencies	1.0
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] Recorded liability represents the estimated minimum probable obligations, as no other amount within the range was a better estimate.