Retirement Benefits (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) bond	Dec. 31, 2017 USD ($)	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 481	$ 551
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.75%
Number of high-quality corporate bonds | bond	600
Defined benefit plan, estimated future employer contributions during the next five years	$ 200
Amortization basis, straight line, in years	10 years
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (560)	$ (534)
Expected return on plan assets	7.00%	7.00%	7.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 79	$ (17)
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	$ 20
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	$ 70
Union Electric Company | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	30.00%
Ameren Illinois Company | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	60.00%