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, 2013		Mar. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 264		$ 246
Operating expenses	(415)	[1]	(826)	[2]
Operating (loss)	(151)		(580)
Other income (loss)	(2)
Interest charges	(11)		(15)
Loss before income taxes	(164)		(595)
Income tax (expense) benefit	(35)		153
Loss from discontinued operations, net of taxes	(199)		(442)
New Ameren Energy Resources Company, LLC [Member]
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
Duck Creek Energy Center [Member] | New Ameren Energy Resources Company, LLC [Member]
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 $155 million 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.