Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 2
2020	8
2021	8
2022	7
2023	6
Thereafter	11
Total lease payments	42
Less imputed interest	4
Total	38
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	2
2020	8
2021	7
2022	6
2023	6
Thereafter	10
Total lease payments	39
Less imputed interest	3
Total	$ 36