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	$ 40,000,000.0	$ 0
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	500,000,000	500,000,000
Amount outstanding at period end	40,000,000	0
Average amount outstanding	64,000,000	72,000,000
Maximum amount outstanding	$ 146,000,000	$ 316,000,000
Weighted average interest rate, computed on a daily basis	1.94%	2.68%
Weighted average interest rate at period end	2.20%