Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 95,000,000	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	48,000,000	0
Average amount outstanding	12,000,000	0
Maximum amount outstanding	$ 97,000,000	$ 20,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.64%	0.92%
Weighted average interest rate at period end (percentage)	1.64%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000
Amount outstanding at period end	95,000,000	0
Average amount outstanding	50,000,000	54,000,000
Maximum amount outstanding	$ 151,000,000	$ 268,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.82%	1.08%
Weighted average interest rate at period end (percentage)	2.28%