Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Amounts recognized in the balance sheet consist of:
Noncurrent liability	$ 1,178		$ 1,344
Net liability recognized	1,183	[1]
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	3,929	[1]	3,645	[1]
Change in benefit obligation:
Net benefit obligation at beginning of year	3,865	[1]	3,451	[1]
Service cost	83	[1],[2]	75	[1],[2]	68	[2]
Interest cost	170	[1],[2]	180	[1],[2]	185	[2]
Plan amendments	(6)	[1],[3],[4]	(16)	[1],[3],[4]
Participant contributions	0	[1]	0	[1]
Actuarial loss	246	[1]	348	[1]
Curtailments(e)	2	[2],[5]	0	[5]
Benefits paid	(209)	[1]	(173)	[1]
Net benefit obligation at end of year	4,151	[1]	3,865	[1]	3,451	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	2,876	[1]	2,722	[1]
Actual return on plan assets	392	[1]	224	[1]
Employer contributions	134	[1]	103	[1]	81	[1]
Participant contributions	0	[1]	0	[1]
Benefits paid	(209)	[1]	(173)	[1]
Fair value of plan assets at end of year	3,193	[1]	2,876	[1]	2,722	[1]
Funded status - deficiency	958	[1]	989	[1]
Accrued benefit cost at December 31	958	[1]	989	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent asset	0		0
Current liability	3	[1]	3	[1]
Noncurrent liability	955	[1]	986	[1]
Net liability recognized	958	[1]	989	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial loss	699	[1]	734	[1]
Prior service cost (credit)	(6)	[1]	(7)	[1]
Transition obligation	0	[1]	0	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	89	[1]	79	[1]
Prior service cost (credit)	(17)	[1]	(15)	[1]
Total	765	[1]	791	[1]
Postretirement Benefits [Member]
Change in benefit obligation:
Net benefit obligation at beginning of year	1,257	[1]	1,120	[1]
Service cost	24	[1],[2]	22	[1],[2]	20	[2]
Interest cost	52	[1],[2]	58	[1],[2]	62	[2]
Plan amendments	(75)	[1]	0	[1]
Participant contributions	16	[1]	18	[1]
Actuarial loss	5	[1]	96	[1]
Curtailments(e)	(1)	[2],[5]	0	[5]
Benefits paid	(73)	[1]	(66)	[1]
Early retiree reinsurance program receipt	2	[1]	3	[1]
Federal subsidy on benefits paid	4	[1]	6	[1]
Net benefit obligation at end of year	1,211	[1]	1,257	[1]	1,120	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	896	[1]	797	[1]
Actual return on plan assets	110	[1]	9	[1]
Employer contributions	45	[1]	129	[1]	36	[1]
Federal subsidy on benefits paid	4	[1]	6	[1]
Early retiree reinsurance program receipt	2	[1]	3	[1]
Participant contributions	16	[1]	18	[1]
Benefits paid	(73)	[1]	(66)	[1]
Fair value of plan assets at end of year	1,000	[1]	896	[1]	797	[1]
Funded status - deficiency	211	[1]	361	[1]
Accrued benefit cost at December 31	211	[1]	361	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent asset	(14)		0
Current liability	2	[1]	3	[1]
Noncurrent liability	223	[1]	358	[1]
Net liability recognized	211	[1]	361	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial loss	103	[1]	177	[1]
Prior service cost (credit)	(24)	[1]	(28)	[1]
Transition obligation	0	[1]	2	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	51	[1]	43	[1]
Prior service cost (credit)	(65)	[1]	(7)	[1]
Total	$ 65	[1]	$ 187	[1]

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

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries

[3] 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.

[4] In 2012, EEI's pension plan was amended to adjust the calculation of the future benefit obligation for all of its active employees from a traditional, final pay formula to a cash balance formula. Additionally, in 2012, EEI's management and labor union postretirement medical benefit plans were amended to adjust for moving to a Medicare Advantage plan.

[5] EEI implemented an employee reduction program in 2012, which resulted in a curtailment of EEI's pension and management postretirement benefit plans.