2010 Corporate Reorganization (Details) (Ameren Illinois Company [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations, net of tax		$ 0	$ 40
Ameren Energy Resources Generating Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues			274
Operating expenses			201
Operating income			73
Other income			1
Interest charges			14
Income taxes			20
Income from discontinued operations, net of tax			$ 40