Retirement Benefits (Details) (USD $) In Millions		1 Months Ended			3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended
	Sep. 30, 2011	Aug. 31, 2011 Ameren Illinois Company [Member]	Sep. 30, 2011 Minimum [Member]	Sep. 30, 2011 Maximum [Member]	Sep. 30, 2011 Pension Benefits [Member]		Sep. 30, 2010 Pension Benefits [Member]		Sep. 30, 2011 Pension Benefits [Member]		Sep. 30, 2010 Pension Benefits [Member]		Sep. 30, 2011 Postretirement Benefits [Member]		Sep. 30, 2010 Postretirement Benefits [Member]		Sep. 30, 2011 Postretirement Benefits [Member]		Sep. 30, 2010 Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost					$ 19	[1]	$ 18	[1]	$ 57	[1]	$ 51	[1]	$ 6	[1]	$ 5	[1]	$ 17	[1]	$ 15	[1]
Interest cost					45	[1]	45	[1]	135	[1]	138	[1]	15	[1]	16	[1]	44	[1]	46	[1]
Expected return on plan assets					(54)	[1]	(53)	[1]	(162)	[1]	(159)	[1]	(14)	[1]	(14)	[1]	(41)	[1]	(42)	[1]
Amortization of transition obligation						[1]		[1]		[1]		[1]	1	[1]	1	[1]	2	[1]	2	[1]
Amortization of prior service cost (benefit)						[1]	1	[1]	(1)	[1]	5	[1]	(2)	[1]	(2)	[1]	(6)	[1]	(6)	[1]
Amortization of actuarial loss (gain)					10	[1]	5	[1]	31	[1]	14	[1]	1	[1]		[1]	3	[1]	1	[1]
Net periodic benefit cost					20	[1]	16	[1]	60	[1]	49	[1]	7	[1]	6	[1]	19	[1]	16	[1]
Defined benefit plan estimated future employer contributions in each of the next five years			125	150
Defined benefit plan estimated future employer contributions over the next five years	690
Additional postretirement benefit plan contribution		$ 100

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