Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 132,000,000	$ 0
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	24,000,000	13,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.84%	1.12%
Weighted average interest rate at period end (percentage)	1.85%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	132,000,000	0
Average amount outstanding	32,000,000	69,000,000
Maximum amount outstanding	$ 140,000,000	$ 176,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.25%	1.13%
Weighted average interest rate at period end (percentage)	2.29%