Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	[2]	$ 111	[1]	$ 99	[1]	$ 156
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	[2]	18	[1]	6	[1]	25
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		47		41		60
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		8		3		10
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		45		39		50
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		8		2		11
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		19		19		46
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans		$ 2		$ 1		$ 4

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

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