Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 22,000,000	$ 11,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	22,000,000	11,000,000
Average amount outstanding	18,000,000	52,000,000
Maximum amount outstanding	$ 46,000,000	$ 129,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.80%	1.23%
Weighted average interest rate at period end (percentage)	2.24%	1.73%