Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended
	Mar. 14, 2013	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2013 center	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transaction Agreement, Closing, Extension Period	1 month
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		$ 13,000,000	$ 155,000,000	$ 168,000,000
Discontinued operations deferred tax expense		1,000,000	98,000,000	97,000,000
Discontinued operations deferred tax benefit		6,000,000	63,000,000	69,000,000
Number of energy centers impaired				3
Disposal group, fair value					133,000,000
Obligation to provide credit support, period	24 months
Buyer's indemnification guarantee obligation	25,000,000
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	5,000,000
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		27,000,000		27,000,000
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
Cash Collateral for Borrowed Securities		29,000,000		29,000,000
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		825,000,000		825,000,000
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		37,000,000		37,000,000
Defined benefit plan assets assumed by counterparty		15,000,000		15,000,000
Transaction agreement, cash retained by counterparty		85,000,000		85,000,000
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	100,000,000
Significant Acquisitions and Disposals, Number Appraisers	3
Expected additional proceeds from sale of assets, minimum	33,000,000
Expected proceeds from sale of assets, minimum	$ 133,000,000
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