Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	[1]	$ 567	$ 710
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		236	277
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		$ 219	$ 278

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