Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended						1 Months Ended	3 Months Ended
	Sep. 30, 2014	Sep. 30, 2014 New Ameren Energy Resources Company, LLC	Sep. 30, 2013 New Ameren Energy Resources Company, LLC	Mar. 14, 2013 New Ameren Energy Resources Company, LLC	Sep. 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	Sep. 30, 2014 Ameren Energy Marketing Company [Member]	Sep. 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			175
Discontinued operations deferred tax expense			96
Discontinued operations deferred tax benefit			72
Working Capital and Contingent Liability Payment		13
Range of possible loss maximum		29.0
Notes, Loans and Financing Receivable, Net, Noncurrent					23	18
Collateral to be posted if credit ratings are below investment grade		23
Buyer's indemnification guarantee obligation				25
Loss contingency accrual		29
Guarantees Outstanding		141							132	9
Guarantees, Maximum Exposure									4	8
Letters of Credit Outstanding, Amount		$ 9