Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 57	[1]	$ 111	[1]	$ 99
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2	[1]	18	[1]	6
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	21		47		41
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1		8		3
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	30		45		39
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1		8		2
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	6		19		19
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 0		$ 2		$ 1

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