Borrowings and Other Financing Instruments Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 307,000,000.0
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000
Amount outstanding at period end	0	307,000,000
Average amount outstanding	129,000,000	55,000,000
Maximum amount outstanding	$ 379,000,000	$ 309,000,000
Weighted average interest rate, computed on a daily basis	2.52%	2.28%
Weighted average interest rate at period end		2.95%