Retirement Benefits (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) bond	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ 551	$ 774
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.50%
Number of high-quality corporate bonds | bond	600
Defined benefit plan, estimated future employer contributions during the next five years	$ 120
Amortization basis, straight line, in years	10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ (534)	$ (705)
Expected return on plan assets	7.00%	7.00%	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ (17)	$ (69)
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	$ 1
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	$ 60
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%