Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
		Sep. 30, 2021	Jun. 30, 2021	Sep. 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				0	30,000,000
Maximum amount outstanding				0	$ 95,000,000
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		0		$ 0	0
Average amount outstanding		11,000,000			$ 0
Maximum amount outstanding		$ 60,000,000	$ 0
Weighted average interest rate, computed on a daily basis		0.05%

[1] Expires in June 2024.