Retirement Benefits (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Defined benefit plan estimated future employer contributions over the next five years	$ 470				$ 470
Minimum [Member]
Defined benefit plan estimated future employer contributions in each of the next five years	75				75
Maximum [Member]
Defined benefit plan estimated future employer contributions in each of the next five years	110				110
Pension Benefits [Member]
Service cost	18	[1]	16	[1]	38	[1]	33	[1]
Interest cost	45	[1]	46	[1]	90	[1]	93	[1]
Expected return on plan assets	(54)	[1]	(53)	[1]	(108)	[1]	(106)	[1]
Amortization of transition obligation		[1]		[1]		[1]		[1]
Amortization of prior service cost (benefit)	(1)	[1]	2	[1]	(1)	[1]	4	[1]
Amortization of actuarial loss (gain)	10	[1]	4	[1]	21	[1]	9	[1]
Net periodic benefit cost	18	[1]	15	[1]	40	[1]	33	[1]
Postretirement Benefits [Member]
Service cost	5	[1]	5	[1]	11	[1]	10	[1]
Interest cost	14	[1]	14	[1]	29	[1]	30	[1]
Expected return on plan assets	(13)	[1]	(14)	[1]	(27)	[1]	(28)	[1]
Amortization of transition obligation	1	[1]	1	[1]	1	[1]	1	[1]
Amortization of prior service cost (benefit)	(2)	[1]	(2)	[1]	(4)	[1]	(4)	[1]
Amortization of actuarial loss (gain)	1	[1]	(1)	[1]	2	[1]	1	[1]
Net periodic benefit cost	$ 6	[1]	$ 3	[1]	$ 12	[1]	$ 10	[1]

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