Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	[1]	$ 774	$ 567
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		293	236
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		$ 315	$ 219

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