Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 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	37,000,000	28,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.10%	0.67%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	0	50,000,000
Average amount outstanding	36,000,000	43,000,000
Maximum amount outstanding	$ 106,000,000	$ 140,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.37%	0.67%
Weighted average interest rate at period end (percentage)		0.95%