Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 42,000,000.0
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	13,000,000	29,000,000
Maximum amount outstanding	$ 89,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis	2.45%	1.96%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500,000,000	$ 400,000,000
Amount outstanding at period end	0	42,000,000
Average amount outstanding	202,000,000	30,000,000
Maximum amount outstanding	$ 316,000,000	$ 144,000,000
Weighted average interest rate, computed on a daily basis	2.68%	2.27%
Weighted average interest rate at period end		2.80%