Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 10,000,000	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	0	0
Average amount outstanding	0	13,000,000
Maximum amount outstanding	$ 1,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.64%	1.12%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	10,000,000	0
Average amount outstanding	4,000,000	69,000,000
Maximum amount outstanding	$ 28,000,000	$ 176,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.86%	1.13%
Weighted average interest rate at period end (percentage)	2.25%