Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended
	Mar. 14, 2013	Mar. 31, 2013 center	Dec. 31, 2012
New Ameren Energy Resources Company, LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		$ 155
Discontinued operations deferred tax liability		98
Discontinued operations deferred tax asset		63
Number of energy centers impaired		3
Disposal group, fair value			133
Obligation to provide credit support, period	24 months
Buyer's indemnification guarantee obligation	25
Buyer's indemnification guarantee obligation, period	24 months
Period for working capital adjustment	120 days
Transitional services to be provided to buyer, period	6 months
Transitional services to be provided to buyer at no charge, period	90 days
Transitional services to be provided to buyer at no charge, maximum amount	5
Transitional services to be provided to buyer, additional period	6 months
Meredosia and Hutsonville Energy Centers [Member] | Ameren Energy Generating Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligation		27
Notes Payable, Other Payables [Member] | New Ameren Energy Resources Company, LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Converted instrument, period for conversion	24 months
Senior Notes [Member] | Ameren Energy Generating Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Aggregate principal amount of senior notes		825
Illinois Power Holdings, LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pension and other postretirement obligations assumed by counterparty		39
Defined benefit plan assets assumed by counterparty		15
Transaction agreement, cash retained by counterparty		85
Elgin, Gibson City and Grand Tower Energy Centers [Member] | Ameren Energy Generating Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of property, plant, and equipment	100
Expected additional proceeds from sale of assets, minimum	33
Expected proceeds from sale of assets, minimum	$ 133
Maximum [Member] | New Ameren Energy Resources Company, LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transitional services to be provided to buyer, period	12 months