Commitments and Contingencies - Leases (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	[1]	$ 30	$ 28
Finance Lease, Right-of-Use Asset, Amortization		2	3
Finance Lease, Interest Expense		4	5
Finance Lease, Cost		6	8
Lessee, Operating Lease, Liability, Payments, Due		582
Total minimum obligation		478
Interest component of obligation		(77)
Finance Lease, Liability, Undiscounted Excess Amount		(348)
Operating Lease, Liability		505
Finance Lease, Liability		130
Less current portion		(100)		$ (97)
Less current portion		(6)
Operating lease liabilities		405		$ 432
Noncurrent operating and finance lease liabilities		124
PPAs
Lessee, Lease, Description [Line Items]
Operating Lease, Cost		26	25
Lessee, Operating Lease, Liability, Payments, Due		518
Interest component of obligation		(70)
Operating Lease, Liability		448
Other
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	[2]	4	$ 3
Lessee, Operating Lease, Liability, Payments, Due		64
Interest component of obligation		(7)
Operating Lease, Liability		$ 57

[1] PPA capacity payments are included in electric fuel and purchased power on the consolidated statements of income. Expense for other operating leases is included in O&M expense and electric fuel and purchased power.

[2] Includes immaterial short-term lease expense for 2021 and 2020.