Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,025,000,000	$ 814,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	3,250,000,000	3,250,000,000
Amount outstanding at period end	1,025,000,000	814,000,000
Average amount outstanding	1,000,000,000	644,000,000
Maximum amount outstanding	$ 1,197,000,000	$ 1,247,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.93%	1.35%
Weighted average interest rate at period end (percentage)	2.34%	1.90%