Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 111,000,000	$ 50,000,000
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	2,000,000	28,000,000
Maximum amount outstanding	$ 31,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.93%	0.67%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	111,000,000	50,000,000
Average amount outstanding	107,000,000	43,000,000
Maximum amount outstanding	$ 150,000,000	$ 140,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.96%	0.67%
Weighted average interest rate at period end (percentage)	1.17%	0.95%