Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended			0 Months Ended	1 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended		3 Months Ended			1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011 Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Dec. 31, 2013 Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Dec. 31, 2012 Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Mar. 14, 2013 New Ameren Energy Resources Company, LLC	Jan. 31, 2014 New Ameren Energy Resources Company, LLC	Dec. 31, 2013 New Ameren Energy Resources Company, LLC	Dec. 31, 2013 Notes Payable, Other Payables New Ameren Energy Resources Company, LLC	Dec. 31, 2013 Elgin, Gibson City and Grand Tower Energy Centers	Dec. 31, 2013 Merchant Generation	Dec. 31, 2012 Merchant Generation	Mar. 31, 2012 Merchant Generation Duck Creek Energy Center	Dec. 31, 2013 Ameren Energy Marketing Company	Dec. 31, 2013 Guarantee Type, Other [Member]	Jan. 31, 2014 Subsequent Event Elgin, Gibson City and Grand Tower Energy Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations	$ 369	$ 349		$ 31	$ 26
Disposal Group, Including Discontinued Operation, Working Capital Adjustment, Period							120 days
Converted instrument, period for conversion									24 months
Notes, Loans and Financing Receivable, Net, Noncurrent									18
Transaction agreement, cash retained by counterparty						235
Obligation to provide credit support, period						24 months
Buyer's indemnification guarantee obligation						25
Buyer's indemnification guarantee obligation, period						24 months
Impairment of assets to be disposed of											201	1,950	628
Disposal group, fair value										137.5
Discontinued operations deferred tax expense											99
Discontinued operations deferred tax benefit											86
Related Party Transaction, Guarantees Outstanding	190													176	14
Related Party Transaction, Guarantees, Maximum Exposure														6
Letters of Credit Outstanding, Amount	11
Loss contingency range of possible loss, maximum								39.0
Other deferred credits and liabilities	622	643						29
Accounts Payable and Accrued Liabilities, Current	806	533						10
Proceeds from Sales of Business, Affiliate and Productive Assets										137.5						168.0
Impairment of Long-Lived Assets Held-for-use			26
Inventory Write-down			4
Proceeds from Asset Sale Held in Escrow																$ 17