Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Fair Value Measurements [Abstract]
Transfers into Level 3/Transfers out of Level 1			$ (1)	[1]
Transfers out of Level 3 / Transfers into Level 1	(30)	[1]
Transfers into Level 3/Transfers out of Level 2			(1)	[1]
Transfers out of Level 3/Transfers into Level 2	(1)	[1]	(32)	[1]
Net fair value of Level 3 transfers	$ (31)	[1]	$ (34)	[1]

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