Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 27	[1]	$ 18	[1]	$ 57	[1]	$ 40	[1]
Pension Plans, Defined Benefit [Member] | Ameren Missouri [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	16		12		32		26
Pension Plans, Defined Benefit [Member] | Ameren Illinois Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	8		3		18		8
Pension Plans, Defined Benefit [Member] | Genco [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	2		3		5		5
Pension Plans, Defined Benefit [Member] | Other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1				2		1
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	2	[1]	6	[1]	11	[1]	12	[1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Ameren Missouri [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			2		5		5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Ameren Illinois Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			4		2		6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Genco [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 2				$ 4		$ 1

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