Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 4
2020	7
2021	7
2022	6
2023	5
Thereafter	10
Total lease payments	39
Less imputed interest	4
Total	35
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	4
2020	7
2021	6
2022	5
2023	5
Thereafter	9
Total lease payments	36
Less imputed interest	4
Total	$ 32