Commitments And Contingencies (Environmental Matters - Remediation) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Manufactured Gas Plant [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss minimum	$ 102.0
Loss contingency range of possible loss maximum	181
Accrual for environmental loss contingencies	102.0	[1]
Ameren Illinois Company [Member] | Manufactured Gas Plant [Member]
Loss Contingencies [Line Items]
Number of remediation sites	44
Loss contingency range of possible loss minimum	99.0
Loss contingency range of possible loss maximum	177
Accrual for environmental loss contingencies	99.0	[1]
Ameren Illinois Company [Member] | Former Coal Ash Landfill [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss minimum	0.5
Loss contingency range of possible loss maximum	6
Accrual for environmental loss contingencies	0.5
Ameren Illinois Company [Member] | Other Environmental [Member]
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.8
Ameren Missouri [Member] | Manufactured Gas Plant [Member]
Loss Contingencies [Line Items]
Number of remediation sites	10
Loss contingency range of possible loss minimum	3.0
Loss contingency range of possible loss maximum	4
Accrual for environmental loss contingencies	3.0	[1]
Ameren Missouri [Member] | Manufactured Gas Plant [Member] | Iowa [Member]
Loss Contingencies [Line Items]
Number of remediation sites	1
Ameren Missouri [Member] | Former Coal Tar Distillery [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss minimum	2.0
Loss contingency range of possible loss maximum	5
Accrual for environmental loss contingencies	2.0
Ameren Missouri [Member] | Sauget Area 2 [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss minimum	0.3
Loss contingency range of possible loss maximum	10
Accrual for environmental loss contingencies	$ 0.3

[1] Recorded liability represents the estimated minimum probable obligations, as no other amount within the range was a better estimate.