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				3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013 New Ameren Energy Resources Company, LLC [Member]	Mar. 31, 2013 New Ameren Energy Resources Company, LLC [Member]	Jun. 30, 2013 New Ameren Energy Resources Company, LLC [Member]	Jun. 30, 2012 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]
Operating revenues	$ 303	$ 258	$ 567		$ 504
Operating expenses	(310)	(238)	(725)	[1]	(1,064)	[2]
Operating income (loss)	(7)	20	(158)		(560)
Other income (loss)	1		(1)		0
Interest charges	(11)	(14)	(22)		(29)
Income (loss) before income taxes	(17)	6	(181)		(589)
Income tax (expense) benefit	7	42	(28)		195
Income (Loss) from discontinued operations, net of taxes	(10)	48	(209)		(394)
Impairment of Long-Lived Assets to be Disposed of							$ 13	$ 155	$ 168	$ 628

[1] Includes a noncash pretax 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.

[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.