Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 0	$ 303	$ 1	$ 567
Operating expenses	(1)	(310)	(3)	(725)	[1]
Operating loss	(1)	(7)	(2)	(158)
Other income (loss)		1		(1)
Interest charges	0	(11)		(22)
Loss before income taxes	(1)	(17)	(2)	(181)
Income tax (expense) benefit	0	7		(28)
Loss from discontinued operations, net of taxes	(1)	(10)	(2)	(209)
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				$ 168

[1] Included a noncash pretax asset impairment charge of $168 million for the six months ended June 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.