Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Dec. 31, 2014	Mar. 14, 2013
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Range of possible loss maximum			$ 25.0
Notes, Loans and Financing Receivable, Net, Current			8.0	$ 12.0
Collateral to be posted if credit ratings are below investment grade			21.0
Buyer's indemnification guarantee obligation					$ 25.0
Loss contingency accrual			25.0
Guarantees Outstanding			74.0
Letters of Credit Outstanding, Amount			9.0
Elgin, Gibson City and Grand Tower Energy Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets	$ 168.0	$ 137.5
Proceeds from Asset Sale Held in Escrow			14.0
Ameren Energy Marketing Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure			$ 6.0