Supplemental Information (Leases - Supplemental Balance Sheet Information) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 38
Operating Lease, Right-of-Use Asset	36
Operating Lease, Liability, Current	7
Operating Lease, Liability, Noncurrent	$ 29
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.60%
Union Electric Company
Lessee, Lease, Description [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 36
Lessee, Operating Lease, Renewal Term	5 years
Operating Lease, Right-of-Use Asset	$ 34
Operating Lease, Liability, Current	6
Operating Lease, Liability, Noncurrent	$ 28
Operating Lease, Weighted Average Remaining Lease Term	6 years
Operating Lease, Weighted Average Discount Rate, Percent	3.60%