Retirement Benefits (Narrative) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016 USD ($) bond	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ 774	$ 567
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate		0.50%
Number of high-quality corporate bonds | bond		700
Defined benefit plan, estimated future employer contributions during the next five years		$ 290
Amortization basis, straight line, in years		10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (705)	$ (544)
Expected return on plan assets		7.00%	7.25%	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (69)	$ (23)
Expected return on plan assets		7.00%	7.00%	7.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years		$ 50
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years		$ 70
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		35.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		55.00%

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