Commitments and Contingencies, Leases - Components of Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Operating leases
Total operating lease expense	$ 61	$ 122
Finance leases
Amortization of ROU assets	2	3
Interest expense on lease liability	5	9
Total finance lease expense	7	12
Short-term lease expense	2	3
Noncurrent operating lease liabilities	1,647	1,647	$ 0
PPAs
Operating leases
Total operating lease expense	53	105
Other
Operating leases
Total operating lease expense	8	17
Mankato Energy Center [Member] | NSP Minnesota [Member] | PPAs
Finance leases
Noncurrent operating lease liabilities	$ 428	$ 428