Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 24		$ 21
Interest cost	44		49
Expected return on plan assets	(62)		(57)
Prior service cost (benefit)			0
Actuarial loss	18		12
Net periodic benefit cost	24	[1]	25	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5		5
Interest cost	12		13
Expected return on plan assets	(17)		(16)
Prior service cost (benefit)	(1)		(1)
Actuarial loss	1		(1)
Net periodic benefit cost	$ 0	[1]	$ 0	[1]

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