Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 311	$ 293	$ 878		$ 797
Operating expenses	(309)	(237)	(1,034)	[1]	(1,301)	[2]
Operating income (loss)	2	56	(156)		(504)
Other income (loss)			(1)
Interest charges	(9)	(14)	(31)		(43)
Income (loss) before income taxes	(7)	42	(188)		(547)
Income tax (expense) benefit	4	21	(24)		216
Income (Loss) from discontinued operations, net of taxes	(3)	63	(212)		(331)
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of Long-Lived Assets to be Disposed of	7		175
Duck Creek Energy Center [Member] | New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of Long-Lived Assets to be Disposed of					$ 628

[1] Includes a noncash pretax impairment charge of $175 million for the nine months ended September 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.

[2] Includes a noncash pretax asset impairment charge of $628 million to reduce the carrying value of AERG’s Duck Creek energy center to its estimated fair value under held and used accounting guidance.