Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Amount outstanding at period end		$ 13,000,000	$ 19,000,000
Credit Facility
Short-term Debt [Line Items]
Long-term Line of Credit		0	0
Line of Credit Facility, Maximum Borrowing Capacity	[1]	150,000,000
Payable to Affiliates
Short-term Debt [Line Items]
Notes Payable, Related Parties, Current		0	0
Commercial Paper
Short-term Debt [Line Items]
Average amount outstanding		9,000,000	30,000,000
Maximum amount outstanding		$ 33,000,000	$ 95,000,000
Maximum amount outstanding		0.17%	1.59%
Weighted average interest rate, computed on a daily basis		0.17%	1.59%
Weighted average interest rate at period end			0.17%
Line of Credit Facility, Maximum Borrowing Capacity		$ 150,000,000	$ 150,000,000
Amount outstanding at period end		0	19,000,000
Money Pool
Short-term Debt [Line Items]
Average amount outstanding		4,000,000	0
Maximum amount outstanding		$ 22,000,000	0
Maximum amount outstanding		0.06%
Weighted average interest rate, computed on a daily basis		0.06%
Weighted average interest rate at period end		0.06%
Line of Credit Facility, Maximum Borrowing Capacity		$ 150,000,000	0
Amount outstanding at period end		$ 13,000,000	$ 0

[1] Expires in June 2024.