Goodwill, Impairment And Other Charges (Reconciliation Of Goodwill) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2010		Dec. 31, 2011	Dec. 31, 2009
Gross goodwill				$ 831	[1]
Accumulated impairment losses		[1]
Goodwill, net of accumulated impairment losses	831	[1]
Impairment losses during year	420	[1],[2]
Goodwill, net of impairment losses	411	[1]	411
Ameren Illinois Company [Member]
Gross goodwill	411			411
Accumulated impairment losses
Goodwill, net of accumulated impairment losses	411		411
Goodwill, net of impairment losses	411		411
Merchant Generation [Member]
Gross goodwill				420
Accumulated impairment losses
Goodwill, net of accumulated impairment losses	420
Impairment losses during year	$ 420

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

[2] Includes amounts for registrant and nonregistrant subsidiaries.