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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 0	$ 311	$ 1	$ 878
Operating expenses	(1)	(309)	(4)	(1,034)	[1]
Operating income (loss)	(1)	2	(3)	(156)
Other income (loss)				(1)
Interest charges	0	(9)		(31)
Loss before income taxes	(1)	(7)	(3)	(188)
Income tax (expense) benefit	0	4		(24)
Loss from discontinued operations, net of taxes	(1)	(3)	(3)	(212)
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				$ 175

[1] Included a noncash pretax asset 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.