Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 20	[1]	$ 18	[1]	$ 41	[1]	$ 38	[1]
Interest cost	42	[1]	45	[1]	85	[1]	90	[1]
Expected return on plan assets	(53)	[1]	(54)	[1]	(107)	[1]	(108)	[1]
Amortization of prior service cost (benefit)	(1)	[1]	(1)	[1]	(1)	[1]	(1)	[1]
Amortization of actuarial loss	19	[1]	10	[1]	39	[1]	21	[1]
Net periodic benefit cost	27	[1]	18	[1]	57	[1]	40	[1]
Postretirement Benefit Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6	[1]	5	[1]	12	[1]	11	[1]
Interest cost	12	[1]	14	[1]	26	[1]	29	[1]
Expected return on plan assets	(15)	[1]	(13)	[1]	(29)	[1]	(27)	[1]
Amortization of Transition obligation	1	[1]	1	[1]	1	[1]	1	[1]
Amortization of prior service cost (benefit)	(2)	[1]	(2)	[1]	(3)	[1]	(4)	[1]
Amortization of actuarial loss			1	[1]	4	[1]	2	[1]
Net periodic benefit cost	$ 2	[1]	$ 6	[1]	$ 11	[1]	$ 12	[1]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.