Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 19		$ 22		$ 40		$ 46
Interest cost	42		41		91		81
Expected return on plan assets	(57)		(54)		(114)		(108)
Prior service cost (benefit)			(1)				(2)
Actuarial loss	12		24		24		46
Net periodic benefit cost	16		32	[1]	41		63	[1]
Postretirement Benefit Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4		5		9		11
Interest cost	12		11		25		23
Expected return on plan assets	(16)		(15)		(32)		(31)
Prior service cost (benefit)	(1)		(1)		(2)		(2)
Actuarial loss	(2)		2		(3)		4
Net periodic benefit cost	(3)		2	[1]	(3)		5	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(3)	[2]	2	[2]	(3)	[2]	5	[2]
Other Postretirement Benefit Plan, Defined Benefit [Member] | New Ameren Energy Resources Company, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			1
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	16	[2]	32	[2]	41	[2]	63	[2]
Pension Plan, Defined Benefit [Member] | New Ameren Energy Resources Company, LLC
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			$ 3				$ 6

[1] The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).

[2] Includes amounts for Ameren registrants and nonregistrant subsidiaries