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
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 83	[1],[2]	$ 75	[1],[2]	$ 68	[1]
Interest cost	170	[1],[2]	180	[1],[2]	185	[1]
Expected return on plan assets	(213)	[1]	(216)	[1]	(212)	[1]
Transition obligation	0	[1]	0	[1]	0	[1]
Prior service cost	(3)	[1]	(1)	[1]	6	[1]
Actuarial loss	77	[1]	42	[1]	18	[1]
Curtailment loss	2	[1],[3]
Net periodic benefit cost	116	[1]	80	[1]	65	[1]
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	24	[1],[2]	22	[1],[2]	20	[1]
Interest cost	52	[1],[2]	58	[1],[2]	62	[1]
Expected return on plan assets	(60)	[1]	(54)	[1]	(56)	[1]
Transition obligation	2	[1]	2	[1]	2	[1]
Prior service cost	(8)	[1]	(8)	[1]	(8)	[1]
Actuarial loss	9	[1]	5	[1]	1	[1]
Curtailment loss	0	[1],[3]
Net periodic benefit cost	19	[1]	25	[1]	21	[1]
Electric Energy Inc [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment loss	$ 2	[2]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries

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

[3] Includes EEI's pension and management postretirement benefit plans' curtailment loss of $2 million recognized in 2012 as a result of its employee reduction program.