Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Amount outstanding at period end		$ 0		$ 0	$ 19,000,000
Credit Facility
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[1]	150,000,000		150,000,000
Long-term Line of Credit		0		0	0
Payable to Affiliates
Short-term Debt [Line Items]
Notes Payable, Related Parties, Current		0		0	0
Commercial Paper
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		150,000,000		150,000,000	150,000,000
Amount outstanding at period end		0		0	$ 19,000,000
Average amount outstanding			$ 30,000,000	0
Maximum amount outstanding			$ 95,000,000	0
Weighted average interest rate, computed on a daily basis			1.59%
Weighted average interest rate at period end					0.17%
Money Pool
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		150,000,000		150,000,000	$ 0
Amount outstanding at period end		33,000,000		$ 33,000,000	$ 0
Average amount outstanding		20,000,000	$ 0
Maximum amount outstanding		$ 41,000,000	$ 0
Weighted average interest rate, computed on a daily basis		0.04%
Weighted average interest rate at period end		0.02%		0.02%

[1] Expires in June 2024.