Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Benefits [Member]
Service cost			$ 75	[1]	$ 68	[1]	$ 68	[1]
Interest cost			180	[1]	185	[1]	186	[1]
Expected return on plan assets			(216)	[1]	(212)	[1]	(206)	[1]
Amortization of prior service cost (benefit)			(1)	[1]	6	[1]	9	[1]
Amortization of actuarial loss			42	[1]	18	[1]	24	[1]
Net periodic benefit cost	91	[1]	80	[1]	65	[1]	81	[1]
Postretirement Benefits [Member]
Service cost			22	[1]	20	[1]	19	[1]
Interest cost			58	[1]	62	[1]	66	[1]
Expected return on plan assets			(54)	[1]	(56)	[1]	(54)	[1]
Amortization of transition obligation			2	[1]	2	[1]	2	[1]
Amortization of prior service cost (benefit)			(8)	[1]	(8)	[1]	(8)	[1]
Amortization of actuarial loss			5	[1]	1	[1]	9	[1]
Net periodic benefit cost	$ 23	[1]	$ 25	[1]	$ 21	[1]	$ 34	[1]

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