Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2022	Dec. 31, 2021
Short-term Debt [Line Items]
Amount outstanding at period end		$ 48,000,000	$ 83,000,000
Credit Facility
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[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]
Line of Credit Facility, Maximum Borrowing Capacity		150,000,000	150,000,000
Amount outstanding at period end		0	83,000,000
Average amount outstanding		48,000,000	3,000,000
Maximum amount outstanding		$ 123,000,000	$ 83,000,000
Weighted average interest rate, computed on a daily basis		0.23%	0.18%
Weighted average interest rate at period end			0.21%
Money Pool
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 150,000,000	$ 150,000,000
Amount outstanding at period end		48,000,000	0
Average amount outstanding		19,000,000	16,000,000
Maximum amount outstanding		$ 65,000,000	$ 78,000,000
Weighted average interest rate, computed on a daily basis		0.13%	0.05%
Weighted average interest rate at period end		0.19%

[1] Expires in June 2024.