Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 20	$ 24
Interest cost		47	44
Expected return on plan assets		(63)	(62)
Prior service cost (benefit)			0
Actuarial loss		9	18
Net periodic benefit cost	[1],[2]	13	24
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		5	5
Interest cost		12	12
Expected return on plan assets		(18)	(17)
Prior service cost (benefit)		(1)	(1)
Actuarial loss		(3)	1
Net periodic benefit cost	[1],[2]	$ (5)	$ 0

[1] (a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates

[2] (b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.