Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 1	$ 264
Operating expenses	(2)	(415)	[1]
Operating loss	(1)	(151)
Other loss		(2)
Interest charges		(11)
Loss before income taxes	(1)	(164)
Income tax expense		(35)
Loss from discontinued operations, net of taxes	(1)	(199)
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		$ 155

[1] Included a noncash pretax asset impairment charge of $155 million, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.