Summary Of Significant Accounting Policies (Narrative) (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basis Of Presentation And Significant Accounting Policies [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	1.3	1.7	1.2	1.7
Debt Issuance Cost	$ 35		$ 35
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Debt Issuance Cost	15		15
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Debt Issuance Cost	$ 19		$ 19