Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 46,000,000	$ 60,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	46,000,000	60,000,000
Average amount outstanding	26,000,000	15,000,000
Maximum amount outstanding	$ 49,000,000	$ 64,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.15%	0.69%
Weighted average interest rate at period end (percentage)	1.36%	0.95%