Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Plant related	$ 3,811	[1]	$ 3,310	[1]
Deferred intercompany tax gain/basis step-up	3	[1]	2	[1]
Regulatory assets (liabilities), net	73	[1]	67	[1]
Deferred benefit costs	(367)	[1]	(360)	[1]
Purchase accounting	35	[1]	106	[1]
ARO	(37)	[1]	(48)	[1]
Other	(223)	[1]	(120)	[1]
Total net accumulated deferred income tax liabilities	3,295	[1],[2]	2,957	[1],[3]
Ameren Corporation [Member]
Current assets	20
Current liabilities			$ 71

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

[2] Includes $20 million, $8 million and $58 million as current assets recorded in the balance sheet for Ameren, Ameren Missouri and Ameren Illinois, respectively.

[3] Includes $43 million as current assets recorded in the balance sheet for Ameren Illinois. Includes $71 million, $43 million and $12 million as current liabilities recorded in the balance sheets for Ameren, Ameren Missouri and Genco, respectively.