Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Amount outstanding at period end	$ 76,000,000.0	$ 65,000,000.0
payable to affiliates [Member]
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	76,000,000	65,000,000
Average amount outstanding	75,000,000	51,000,000
Maximum amount outstanding	$ 95,000,000	$ 93,000,000
Weighted average interest rate, computed on a daily basis	1.97%	2.38%
Weighted average interest rate at period end	2.52%	1.97%