Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 23	$ 20	$ 69	$ 60
Interest cost		43	46	130	137
Expected return on plan assets		(62)	(58)	(186)	(172)
Prior service cost (benefit)			(1)		(1)
Actuarial loss		19	13	56	37
Settlement loss		0	0	1	0
Net periodic benefit cost	[1],[2]	23	20	70	61
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		6	5	17	14
Interest cost		12	12	36	37
Expected return on plan assets		(17)	(16)	(51)	(48)
Prior service cost (benefit)		(1)	(2)	(3)	(4)
Actuarial loss		1	(2)	4	(5)
Settlement loss		0	0	0	0
Net periodic benefit cost	[1],[2]	$ 1	$ (3)	$ 3	$ (6)

[1] (a)The amounts above do 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 built into rates

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