Supplemental Information (Leases - Maturities of Operating Lease Liabilities) (Details) $ in Millions	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2019	$ 6
2020	7
2021	7
2022	6
2023	5
Thereafter	10
Total lease payments	41
Less imputed interest	5
Total	36
Union Electric Company
Lessee, Lease, Description [Line Items]
2019	5
2020	7
2021	6
2022	5
2023	5
Thereafter	10
Total lease payments	38
Less imputed interest	4
Total	$ 34