Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 50,000,000.0	$ 51,000,000.0
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	50,000,000	51,000,000
Average amount outstanding	39,000,000	28,000,000
Maximum amount outstanding	$ 59,000,000	$ 103,000,000
Weighted average interest rate, computed on a daily basis	2.63%	2.31%
Weighted average interest rate at period end	2.62%	2.89%