Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Current assets of discontinued operations
Cash and cash equivalents	$ 25		$ 25
Accounts receivable and unbilled revenue	102		102
Materials and supplies	119		134
Mark-to-market derivative assets	111		102
Property and plant, net	615		748
Accumulated deferred income taxes, net	380		373
Other assets	134		116
Total current assets of discontinued operations	1,486		1,600
Current liabilities of discontinued operations
Accounts payable and other current obligations	142		133
Mark-to-market derivative liabilities	70		63
Long-term debt, net	824		824
Asset retirement obligations	87		78
Pension and other postretirement benefits	37		40
Other liabilities	23		28
Total current liabilities of discontinued operations	1,183		1,166
Accumulated other comprehensive gain (loss)	8	[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 income related to discontinued operations remains in “Accumulated other comprehensive loss” on Ameren’s June 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 remains in “Noncontrolling interests” on Ameren’s June 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.