Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 25,000,000	$ 15,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	100,000,000	0
Average amount outstanding	50,000,000	21,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.64%	0.40%
Weighted average interest rate at period end (percentage)	0.62%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	25,000,000	15,000,000
Average amount outstanding	71,000,000	100,000,000
Maximum amount outstanding	$ 140,000,000	$ 246,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.67%	0.46%
Weighted average interest rate at period end (percentage)	0.65%	0.60%