Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 49,000,000	$ 51,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	49,000,000	51,000,000
Average amount outstanding	52,000,000	28,000,000
Maximum amount outstanding	$ 71,000,000	$ 103,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.71%	2.31%
Weighted average interest rate at period end (percentage)	2.67%	2.89%