Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended
	Sep. 30, 2014	Dec. 31, 2012	Jan. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Mar. 31, 2012	Dec. 31, 2014	Mar. 14, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations				$ 369	$ 369		$ 396
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations				31	31		32
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		2,580
Working Capital and Contingent Liability Payment	13
Loss contingency range of possible loss, maximum							29.0
Collateral to be posted if credit ratings are below investment grade							26
Buyer's indemnification guarantee obligation								25
Loss Contingency Accrual							29
Guarantees Outstanding							114
Letters of Credit Outstanding, Amount							9
Notes Payable, Other Payables | New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Notes, Loans and Financing Receivable, Net, Noncurrent				18	18		12
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			17
Disposal group, fair value				137.5	137.5
Merchant Generation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		1,950			201
Discontinued operations deferred tax expense					99
Discontinued operations deferred tax benefit					86
Merchant Generation | Duck Creek Energy Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of						628
Ameren Energy Marketing Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure							11
Guarantees Outstanding							106
Guarantee Type, Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure							7
Guarantees Outstanding							$ 8