Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 137,000,000	$ 42,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	38,000,000	0
Average amount outstanding	19,000,000	29,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.44%	1.96%
Weighted average interest rate at period end (percentage)	2.44%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	137,000,000	42,000,000
Average amount outstanding	86,000,000	30,000,000
Maximum amount outstanding	$ 152,000,000	$ 144,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.69%	2.27%
Weighted average interest rate at period end (percentage)	2.71%	2.80%