Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			6 Months Ended					1 Months Ended	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2013 New Ameren Energy Resources Company, LLC	Jun. 30, 2014 New Ameren Energy Resources Company, LLC	Mar. 14, 2013 New Ameren Energy Resources Company, LLC	Jun. 30, 2014 Notes Payable, Other Payables [Member] New Ameren Energy Resources Company, LLC	Dec. 31, 2013 Notes Payable, Other Payables [Member] New Ameren Energy Resources Company, LLC	Jan. 31, 2014 Elgin, Gibson City and Grand Tower Energy Centers	Dec. 31, 2013 Elgin, Gibson City and Grand Tower Energy Centers	Jun. 30, 2014 Ameren Energy Marketing Company [Member]	Jun. 30, 2014 Guarantee Type, Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Escrow Deposit, Reserve Period			2 years
Proceeds from Sales of Business, Affiliate and Productive Assets											$ 168.0	$ 137.5
Proceeds from Asset Sale Held in Escrow											17
Impairment of assets to be disposed of						168
Discontinued operations deferred tax expense						97
Discontinued operations deferred tax benefit						69
Loss from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	1	10	2	209
Asset retirement obligations	380		380		369
Range of possible loss maximum							34.0
Other deferred credits and liabilities	524		524		538		29
Accounts and wages payable	575		575		806		5
Notes, Loans and Financing Receivable, Net, Noncurrent									26	18
Buyer's indemnification guarantee obligation								25
Loss contingency accrual							34
Guarantees Outstanding							147						138	9
Guarantees, Maximum Exposure													10	8
Letters of Credit Outstanding, Amount							$ 9