Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 65,000,000.0
Payable to Affiliates
Short-term Debt [Line Items]
Notes Payable, Related Parties, Current	0	0
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	0	65,000,000
Average amount outstanding	41,000,000	51,000,000
Maximum amount outstanding	$ 91,000,000	$ 93,000,000
Weighted average interest rate, computed on a daily basis	1.04%	2.38%
Weighted average interest rate at period end		1.97%