Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)	0 Months Ended	9 Months Ended				0 Months Ended	3 Months Ended		9 Months Ended			0 Months Ended						0 Months Ended			3 Months Ended	0 Months Ended
	Mar. 14, 2013 D disposal_groups	Sep. 30, 2013		Dec. 31, 2012		Mar. 14, 2013 New Ameren Energy Resources Company, LLC	Sep. 30, 2013 New Ameren Energy Resources Company, LLC	Mar. 31, 2013 New Ameren Energy Resources Company, LLC	Sep. 30, 2013 New Ameren Energy Resources Company, LLC center	Dec. 31, 2012 New Ameren Energy Resources Company, LLC	Sep. 30, 2013 Meredosia and Hutsonville Energy Centers [Member] Ameren Energy Generating Company	Mar. 14, 2013 Notes Payable, Other Payables [Member] New Ameren Energy Resources Company, LLC	Sep. 30, 2013 Notes Payable, Other Payables [Member] New Ameren Energy Resources Company, LLC	Sep. 30, 2013 Senior Notes [Member] Ameren Energy Generating Company	Sep. 30, 2013 Illinois Power Holdings, LLC [Member]	Sep. 30, 2013 Elgin, Gibson City and Grand Tower Energy Centers	Mar. 31, 2013 Elgin, Gibson City and Grand Tower Energy Centers	Mar. 14, 2013 Elgin, Gibson City and Grand Tower Energy Centers Ameren Energy Generating Company appraiser	Mar. 14, 2013 Initial Payment Received [Member]	Mar. 14, 2013 Maximum New Ameren Energy Resources Company, LLC	Jun. 30, 2013 Claimed Manufacturing Exemptions And Credits [Member] Ameren Energy Generating Company	Oct. 11, 2013 Subsequent Event [Member] Elgin, Gibson City and Grand Tower Energy Centers	Oct. 11, 2013 Subsequent Event [Member] Additional Payment Received [Member]	Oct. 11, 2013 Subsequent Event [Member] Total Payment Received [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets																		$ 100,000,000	$ 100,000,000				$ 37,500,000	$ 137,500,000
Escrow Deposit, Reserve Period		2 years
Period to distribution proceeds from sale of business																							2 years
Gain (Loss) Related to Litigation Settlement																					7,000,000
Pension and other postretirement obligations assumed by counterparty															32,000,000
Defined benefit plan assets assumed by counterparty															15,000,000
Asset retirement obligations		392,000,000	[1]	375,000,000	[1]						27,000,000
Converted instrument, period for conversion												24 months
Cash Collateral for Borrowed Securities													27,000,000
Aggregate principal amount of senior notes														825,000,000
Transaction agreement, cash retained by counterparty															85,000,000
Gain (Loss) on Contract Termination	25,000,000
Contract Covenant Compliance, Number of Business Days	3
Significant Acquisitions and Disposals, Number Appraisers																		3
Impairment of assets to be disposed of							7,000,000		175,000,000
Discontinued operations deferred tax expense							1,000,000	96,000,000
Number of Energy Centers Impaired									3
Discontinued operations deferred tax benefit							3,000,000	72,000,000
Disposal group, fair value										133,000,000						137,500,000	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														12 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
SignificantAcquisitionsAndDisposalsExpectedAdditionalSaleProceedsMaximum																						$ 15,000,000
Discontinued Operations, Number of Disposal Groups	2

[1] The nuclear decommissioning trust fund assets of $459 million and $408 million as of September 30, 2013, and December 31, 2012, respectively, were restricted for decommissioning of the Callaway energy center.