Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 10,000	$ 10,000	$ 78,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000	150,000	150,000	$ 150,000
Amount outstanding at period end	10,000	10,000	78,000	68,000
Average amount outstanding	6,000	39,000	46,000	20,000
Maximum amount outstanding	$ 18,000	$ 122,000	$ 101,000	$ 71,000
Weighted average interest rate, computed on a daily basis (percentage)	0.44%	0.44%	0.27%	0.31%
Weighted average interest rate at period end (percentage)	0.70%	0.70%	0.55%	0.27%