Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Income Taxes [Line Items]
Plant related	$ 4,201	[1]	$ 3,826	[1]
Long-lived asset impairments	(986)	[1]	(15)	[1]
Deferred intercompany tax gain/basis step-up	2	[1]	3	[1]
Regulatory assets (liabilities), net	73	[1]	73	[1]
Deferred benefit costs	(337)	[1]	(367)	[1]
Purchase accounting	(10)	[1]	35	[1]
ARO	(44)	[1]	(37)	[1]
Other	(278)	[1],[2]	(223)	[1]
Total net accumulated deferred income tax liabilities	2,621	[1],[3]	3,295	[1],[4]
Current assets	171		20
Union Electric Company [Member]
Income Taxes [Line Items]
Plant related	2,386		2,134
Long-lived asset impairments
Deferred intercompany tax gain/basis step-up	(1)		(1)
Regulatory assets (liabilities), net	73		73
Deferred benefit costs	(84)		(88)
Purchase accounting
ARO	(7)
Other	50	[2]	6
Total net accumulated deferred income tax liabilities	2,417	[3]	2,124	[4]
Current assets	26		8
Ameren Illinois Company [Member]
Income Taxes [Line Items]
Plant related	1,106		1,003
Long-lived asset impairments
Deferred intercompany tax gain/basis step-up	39		55
Regulatory assets (liabilities), net
Deferred benefit costs	(102)		(109)
Purchase accounting	(27)		(27)
ARO	1		1
Other	(77)	[2]	(86)
Total net accumulated deferred income tax liabilities	940	[3]	837	[4]
Current assets	$ 85		$ 58

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

[2] Includes deferred tax assets related to net operating loss and tax credit carryforwards detailed in the table below.

[3] Includes $26 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2012.

[4] Includes $8 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2011.