Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 5,000,000	$ 10,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	5,000,000	10,000,000
Average amount outstanding	14,000,000	39,000,000
Maximum amount outstanding	$ 38,000,000	$ 122,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.66%	0.44%
Weighted average interest rate at period end (percentage)	0.65%	0.70%