Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		9 Months Ended	12 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Amount outstanding at period end		$ 0	$ 65,000,000.0
Credit Facility
Short-term Debt [Line Items]
Borrowing limit	[1]	150,000,000
Long-term Line of Credit		0	0
Payable to Affiliates
Short-term Debt [Line Items]
Notes Payable, Related Parties, Current		0	0
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit		150,000,000	150,000,000
Amount outstanding at period end		0	65,000,000
Average amount outstanding		0	51,000,000
Maximum amount outstanding		$ 0	$ 93,000,000
Weighted average interest rate, computed on a daily basis			2.38%
Weighted average interest rate at period end			1.97%

[1] Expires in June 2024.