Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Noncurrent liability	$ 1,344,000,000		$ 1,045,000,000
Total liability	1,350,000,000	[1]
Pension Benefits [Member]
Accumulated benefit obligation at end of year	3,645,000,000	[1]	3,246,000,000	[1]
Net benefit obligation at beginning of year	3,451,000,000	[1]	3,255,000,000	[1]
Service cost	75,000,000	[1]	68,000,000	[1]	68,000,000	[1]
Interest cost	180,000,000	[1]	185,000,000	[1]	186,000,000	[1]
Plan amendments	(16,000,000)	[1],[2],[3]	(40,000,000)	[1],[2],[3]
Actuarial (gain) loss	348,000,000	[1]	165,000,000	[1]
Benefits paid	(173,000,000)	[1]	(182,000,000)	[1]
Net benefit obligation at end of year	3,865,000,000	[1]	3,451,000,000	[1]	3,255,000,000	[1]
Fair value of plan assets at beginning of year	2,722,000,000	[1]	2,495,000,000	[1]
Actual return on plan assets	224,000,000	[1]	328,000,000	[1]
Employer contributions	103,000,000	[1]	81,000,000	[1]	99,000,000	[1]
Fair value of plan assets at end of year	2,876,000,000	[1]	2,722,000,000	[1]	2,495,000,000	[1]
Funded status - deficiency	989,000,000	[1]	729,000,000	[1]
Accrued benefit cost at December 31	989,000,000	[1]	729,000,000	[1]
Current liability	3,000,000	[1]	4,000,000	[1]
Noncurrent liability	986,000,000	[1]	725,000,000	[1]
Total liability	989,000,000	[1]	729,000,000	[1]
Net actuarial loss	734,000,000	[1]	507,000,000	[1]
Prior service cost (credit)	(7,000,000)	[1]	(11,000,000)	[1]
Amounts recognized in accumulated OCI, Net actuarial loss	79,000,000	[1]	24,000,000	[1]
Amounts recognized in accumulated OCI, Prior service cost (credit)	(15,000,000)	[1]	4,000,000	[1]
Defined benefit plan, accumulated other comprehensive income and regulatory assets, before tax	791,000,000	[1]	524,000,000	[1]
Postretirement Benefits [Member]
Net benefit obligation at beginning of year	1,120,000,000	[1]	1,143,000,000	[1]
Service cost	22,000,000	[1]	20,000,000	[1]	19,000,000	[1]
Interest cost	58,000,000	[1]	62,000,000	[1]	66,000,000	[1]
Participant contributions	18,000,000	[1]	17,000,000	[1]
Actuarial (gain) loss	96,000,000	[1]	(53,000,000)	[1]
Benefits paid	(66,000,000)	[1]	(74,000,000)	[1]
Early retiree reinsurance program receipt	3,000,000	[1]
Federal subsidy on benefits paid	6,000,000	[1]	5,000,000	[1]
Net benefit obligation at end of year	1,257,000,000	[1]	1,120,000,000	[1]	1,143,000,000	[1]
Fair value of plan assets at beginning of year	797,000,000	[1]	732,000,000	[1]
Actual return on plan assets	9,000,000	[1]	81,000,000	[1]
Employer contributions	129,000,000	[1]	36,000,000	[1]	49,000,000	[1]
Fair value of plan assets at end of year	896,000,000	[1]	797,000,000	[1]	732,000,000	[1]
Funded status - deficiency	361,000,000	[1]	323,000,000	[1]
Accrued benefit cost at December 31	361,000,000	[1]	323,000,000	[1]
Current liability	3,000,000	[1]	3,000,000	[1]
Noncurrent liability	358,000,000	[1]	320,000,000	[1]
Total liability	361,000,000	[1]	323,000,000	[1]
Net actuarial loss	177,000,000	[1]	86,000,000	[1]
Prior service cost (credit)	(28,000,000)	[1]	(32,000,000)	[1]
Transition obligation	2,000,000	[1]	5,000,000	[1]
Amounts recognized in accumulated OCI, Net actuarial loss	43,000,000	[1]	13,000,000	[1]
Amounts recognized in accumulated OCI, Prior service cost (credit)	(7,000,000)	[1]	(10,000,000)	[1]
Defined benefit plan, accumulated other comprehensive income and regulatory assets, before tax	$ 187,000,000	[1]	$ 62,000,000	[1]

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

[2] In 2011, Ameren's pension plan was amended to adjust the calculation of the future benefit obligation of approximately 430 labor union-represented employees from a traditional, final pay formula to a cash balance formula.

[3] In 2010, Ameren's pension plan was amended to adjust the calculation of the future benefit obligation of approximately 700 management employees from a traditional, final pay formula to a cash balance formula.