Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Current assets of discontinued operations
Cash and cash equivalents	$ 25		$ 25
Accounts receivable and unbilled revenue	103		102
Materials and supplies	117		134
Mark-to-market derivative assets	101		102
Property and plant, net	607		748
Accumulated deferred income taxes, net	412		373
Other assets	135		116
Total current assets of discontinued operations	1,500		1,600
Current liabilities of discontinued operations
Accounts payable and other current obligations	138		133
Mark-to-market derivative liabilities	82		63
Long-term debt, net	824		824
Asset retirement obligations	86		78
Pension and other postretirement benefits	39		40
Other liabilities	29		28
Total current liabilities of discontinued operations	1,198		1,166
Accumulated other comprehensive gain (loss)	12	[1]	19	[1]
Noncontrolling Interest	$ 8	[2]	$ 8	[2]
Electric Energy, Inc [Member]
Current liabilities of discontinued operations
Percentage of EEI not owned by Ameren	20.00%		20.00%

[1] Accumulated other comprehensive gain related to discontinued operations remains in “Accumulated other comprehensive gain (loss)” on Ameren’s March 31, 2013, and December 31, 2012, balance sheets. This balance relates to New AER assets and liabilities that will be realized or removed from Ameren’s balance sheet either before or at the closing of the New AER divestiture.

[2] The 20% ownership interest of EEI not owned by Ameren remains in “Noncontrolling interests” on Ameren’s March 31, 2013, and December 31, 2012, balance sheets. This noncontrolling interest will be removed from Ameren’s balance sheet at the closing of the New AER divestiture.