Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 20		$ 23		$ 60		$ 69
Interest cost	46		40		137		121
Expected return on plan assets	(58)		(54)		(172)		(162)
Prior service cost (benefit)	(1)		(1)		(1)		(3)
Actuarial loss	13		23		37		69
Net periodic benefit cost	20		31	[1]	61		94	[1]
Postretirement Benefit Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5		6		14		17
Interest cost	12		11		37		34
Expected return on plan assets	(16)		(16)		(48)		(47)
Prior service cost (benefit)	(2)		(1)		(4)		(3)
Actuarial loss	(2)		2		(5)		6
Net periodic benefit cost	(3)		2	[1]	(6)		7	[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]	(6)	[2]	7	[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	[3]			1	[3]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	20	[2]	31	[2]	61	[2]	94	[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			$ 2				$ 8

[1] ncludes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income

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

[3] Less than $1 million.