Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Assets of discontinued operations
Cash and cash equivalents	$ 25		$ 25		$ 25
Accounts receivable and unbilled revenue	102		102
Materials and supplies	121		134
Mark-to-market derivative assets	71		102
Property and plant, net	623		748
Accumulated deferred income taxes, net	357		385
Other assets	96		104
Total assets of discontinued operations	1,395		1,600
Liabilities of discontinued operations
Accounts payable and other current obligations	141		133
Mark-to-market derivative liabilities	38		63
Long-term debt, net	824		824
Asset retirement obligations	87		78
Pension and other postretirement benefits	32		40
Other liabilities	19		28
Total liabilities of discontinued operations	1,141		1,166
Accumulated other comprehensive gain (loss)	3	[1]	19	[1]
Noncontrolling Interest	$ 8	[2]	$ 8	[2]
Electric Energy, Inc
Liabilities of discontinued operations
Percentage of EEI not owned by Ameren	20.00%		20.00%

[1] Accumulated other comprehensive income related to discontinued operations remains in “Accumulated other comprehensive loss” on Ameren’s September 30, 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 was included in “Noncontrolling interests” on Ameren’s September 30, 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.