Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 145	$ 133
Current losses deferred as regulatory assets	236	215
Power [Member]
Derivative [Line Items]
Gain (loss) to be amortized in next year	17.0	5.0
Current gains deferred as regulatory liabilities	37	29
Current losses deferred as regulatory assets	33	17
Interest Rate Contract [Member]
Derivative [Line Items]
Gain (loss) to be amortized in next year	(1.4)
Carrying value of net gains associated with interest rate swaps		1
Carrying value of net losses associated with interest rate swaps	8	9
Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7	16
Current losses deferred as regulatory assets	2	1
Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2	1
Current losses deferred as regulatory assets	88	101
Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	1	1
Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	37	29
Current losses deferred as regulatory assets	14	8
Ameren Missouri [Member] | Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7	16
Current losses deferred as regulatory assets	2	1
Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	10	11
Ameren Missouri [Member] | Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	1	1
Ameren Illinois Company [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	98	76
Current losses deferred as regulatory assets	231	306
Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	133	209
Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2	1
Current losses deferred as regulatory assets	$ 78	$ 90