Retirement Benefits (Schedule Of Matching Contributions) (Details) - 401 (K) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	[1]	$ 29	$ 28	$ 27
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		16	16	16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		12	11	10
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 1	$ 1	$ 1

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.