Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 1,183	[1]
Union Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	464
Ameren Illinois Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 408

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