Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended							3 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2013 Merchant Generation	Dec. 31, 2012 Merchant Generation	Mar. 31, 2012 Merchant Generation Duck Creek Energy Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$ 1,037		$ 1,047		$ 1,358
Operating expenses	(1,207)	[1]	(3,474)	[2]	(1,150)
Operating income (loss)	(170)		(2,427)		208
Other income (loss)	(1)				1
Interest charges	(39)		(56)		(64)
Income (loss) before income taxes	(210)		(2,483)		145
Income tax (expense) benefit	(13)		987		(56)
Income (Loss) from Discontinued Operations, Net of Tax (Note 16)	(223)		(1,496)		89
Impairment of assets to be disposed of						$ 201	$ 1,950	$ 628

[1] (a)Includes a $201 million pretax loss on disposal relating to the New AER divestiture.

[2] (b)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. In addition, includes a noncash pretax asset impairment charge of $1.95 billion to reduce the carrying values of all the AER coal and natural gas-fired energy centers, except the Joppa coal-fired energy center, to their estimated fair values, under held and used accounting guidance, as a result of the decision in December 2012 that Ameren intended to exit the Merchant Generation business.