Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 81	[1]	$ 92	[1]	$ 79
Interest cost	185	[1]	174	[1]	183
Expected return on plan assets	(253)		(248)		(229)
Prior service credit	(1)		(1)		(1)
Actuarial (gain) loss	32		74		49
Settlement Loss			1
Net periodic benefit cost (benefit)	44		92		81
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	[1]	24	[1]	19
Interest cost	50	[1]	48	[1]	50
Expected return on plan assets	(72)		(68)		(65)
Prior service credit	(5)		(5)		(5)
Actuarial (gain) loss	(11)		5		(7)
Settlement Loss			0
Net periodic benefit cost (benefit)	$ (19)		$ 4		$ (8)

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