Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Change in plan assets:
Funded status – deficiency	[1]	$ (774)		$ (567)
Amounts recognized in the balance sheet consist of:
Noncurrent liability		769		580
Net liability recognized	[2]	774		567
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	[1]	4,288		3,995
Change in benefit obligation:
Net benefit obligation at beginning of year	[1]	4,197		4,410
Service cost		81	[1]	92	[1]	$ 79
Interest cost		185	[1]	174	[1]	183
Actuarial (gain) loss	[1]	265		(256)
Settlement	[1]			(2)
Benefits paid	[1]	(210)		(221)
Net benefit obligation at end of year	[1]	4,518		4,197		4,410
Change in plan assets:
Fair value of plan assets at beginning of year	[1]	3,653		3,794
Actual return on plan assets	[1]	313		(29)
Employer contributions		57	[1]	111	[1]	99
Settlement	[1]			(2)
Benefits paid	[1]	(210)		(221)
Fair value of plan assets at end of year	[1]	3,813		3,653		3,794
Funded status – deficiency	[1]	705		544
Accrued benefit cost at December 31	[1]	705		544
Amounts recognized in the balance sheet consist of:
Current liability(d)	[1],[3]	3		3
Noncurrent liability	[1]	702		541
Net liability recognized	[1]	705		544
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	[1]	535		395
Prior service cost (credit)	[1]	(4)		(5)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	[1]	43		17
Prior service cost (credit)	[1]			0
Total	[1]	574		407
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	[1]	1,094		1,203
Service cost		19	[1]	24	[1]	19
Interest cost		50	[1]	48	[1]	50
Participant contributions	[1]	8		8
Actuarial (gain) loss	[1]	52		(133)
Benefits paid	[1]	(54)		(56)
Federal subsidy on benefits paid	[1]	1		0
Net benefit obligation at end of year	[1]	1,170		1,094		1,203
Change in plan assets:
Fair value of plan assets at beginning of year	[1]	1,071		1,109
Actual return on plan assets	[1]	73		(8)
Employer contributions		2	[1]	18	[1]	6
Federal subsidy on benefits paid	[1]	1		0
Participant contributions	[1]	8		8
Benefits paid	[1]	(54)		(56)
Fair value of plan assets at end of year	[1]	1,101		1,071		$ 1,109
Funded status – deficiency	[1]	69		23
Accrued benefit cost at December 31	[1]	69		23
Amounts recognized in the balance sheet consist of:
Noncurrent asset	[1],[4]	0		(18)
Current liability(d)	[1],[3]	2		2
Noncurrent liability	[1]	67		39
Net liability recognized	[1]	69		23
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	[1]	(29)		(82)
Prior service cost (credit)	[1]	(8)		(11)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	[1]	0		(3)
Prior service cost (credit)	[1]	(1)		0
Total	[1]	$ (38)		$ (96)

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