Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2020	$ 8
2021	8
2022	7
2023	6
2024	5
Thereafter	5
Total lease payments	39
Less imputed interest	3
Total	36
Union Electric Company
Lessee, Lease, Description [Line Items]
2020	8
2021	7
2022	6
2023	6
2024	5
Thereafter	5
Total lease payments	37
Less imputed interest	3
Total	$ 34