Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 22	$ 19	$ 46	$ 40
Interest cost		43	42	87	91
Expected return on plan assets		(62)	(57)	(124)	(114)
Prior service cost (benefit)			0		0
Actuarial loss		19	12	37	24
Settlement loss		1	0	1	0
Net periodic benefit cost	[1]	23	16	47	41
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		6	4	11	9
Interest cost		12	12	24	25
Expected return on plan assets		(17)	(16)	(34)	(32)
Prior service cost (benefit)		(1)	(1)	(2)	(2)
Actuarial loss		2	(2)	3	(3)
Net periodic benefit cost	[1]	$ 2	$ (3)	$ 2	$ (3)

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