Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 80,000,000	$ 15,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	0	0
Average amount outstanding	11,000,000	21,000,000
Maximum amount outstanding	$ 88,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.71%	0.40%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	80,000,000	15,000,000
Average amount outstanding	58,000,000	100,000,000
Maximum amount outstanding	$ 93,000,000	$ 246,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.65%	0.46%
Weighted average interest rate at period end (percentage)	0.70%	0.60%