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, 2014		Dec. 31, 2013		Dec. 31, 2012
Change in plan assets:
Funded status – deficiency	$ (710)	[1]	$ (461)	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent liability	705		466
Net liability recognized	710	[1]	461	[1]
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	4,176	[1]	3,698	[1]
Change in benefit obligation:
Net benefit obligation at beginning of year	3,900	[1]	4,051	[1]
Service cost	79	[1],[2]	91	[1],[2]	81	[2]
Interest cost	183	[1],[2]	163	[1],[2]	166	[2]
Actuarial (gain) loss	462	[1]	(207)	[1]
Benefits paid	(214)	[1]	(198)	[1]
Net benefit obligation at end of year	4,410	[1]	3,900	[1]	4,051	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	3,461	[1]	3,127	[1]
Actual return on plan assets	448	[1]	376	[1]
Employer contributions	99	[1]	156	[1]	128	[1]
Benefits paid	(214)	[1]	(198)	[1]
Fair value of plan assets at end of year	3,794	[1]	3,461	[1]	3,127	[1]
Funded status – deficiency	616	[1]	439	[1]
Accrued benefit cost at December 31	616	[1]	439	[1]
Amounts recognized in the balance sheet consist of:
Current liability(f)	3	[1],[3]	3	[1],[3]
Noncurrent liability	613	[1]	436	[1]
Net liability recognized	616	[1]	439	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	452	[1]	282	[1]
Prior service cost (credit)	(6)	[1]	(7)	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	29	[1]	17	[1]
Prior service cost (credit)			0	[1]
Total	475	[1]	292	[1]
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	1,096	[1]	1,157	[1]
Service cost	19	[1],[2]	22	[1],[2]	22	[2]
Interest cost	50	[1],[2]	46	[1],[2]	47	[2]
Participant contributions	16	[1]	16	[1]
Actuarial (gain) loss	84	[1]	(76)	[1]
Curtailment gain(c)			(3)	[2],[4]
Settlement			5	[5]
Benefits paid	(65)	[1]	(64)	[1]
Federal subsidy on benefits paid	3	[1]	3	[1]
Net benefit obligation at end of year	1,203	[1]	1,096	[1]	1,157	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	1,074	[1]	938	[1]
Actual return on plan assets	75	[1]	156	[1]
Employer contributions	6	[1]	25	[1]	45	[1]
Federal subsidy on benefits paid	3	[1]	3	[1]
Participant contributions	16	[1]	16	[1]
Benefits paid	(65)	[1]	(64)	[1]
Fair value of plan assets at end of year	1,109	[1]	1,074	[1]	938	[1]
Funded status – deficiency	94	[1]	22	[1]
Accrued benefit cost at December 31	94	[1]	22	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent asset	0	[1],[6]	9
Current liability(f)	2	[1],[3]	1	[1],[3]
Noncurrent liability	92	[1]	30	[1]
Net liability recognized	94	[1]	22	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	(7)	[1]	(71)	[1]
Prior service cost (credit)	(16)	[1]	(20)	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	(5)	[1]	(12)	[1]
Prior service cost (credit)	(1)	[1]	(1)	[1]
Total	$ (29)	[1]	$ (104)	[1]

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

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries

[3] Included in "Other current liabilities" on Ameren's consolidated balance sheet.

[4] Effective with the divestiture of New AER on December 2, 2013, the liability for active management employees of New AER and its subsidiaries not eligible to retire were neither transferred to IPH nor retained by Ameren, which resulted in a curtailment gain. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.

[5] Not applicable.

[6] Included in "Other assets" on Ameren's consolidated balance sheet.