Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Pension Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 31	[1]	$ 28	[1]	$ 94	[1]	$ 83	[1]
Pension Benefit [Member] | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	18		16		54		48
Pension Benefit [Member] | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	10		9		31		27
Pension Benefit [Member] | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	3		3		9		8
Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	2	[1]	4	[1]	7	[1]	11	[1]
Postretirement Benefits [Member] | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	2		2		7		7
Postretirement Benefits [Member] | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	[2]	1		1	[2]	3
Postretirement Benefits [Member] | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 1	[2]	$ 1		$ 1	[2]	$ 1

[1] Includes amounts for Ameren registrants and nonregistrant subsidiaries, but excludes the EEI plans as they are included in discontinued operations.

[2] Less than $1 million.