Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 710	[1]	$ 461	[1]
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	277		191
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 278		$ 159

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