Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended	12 Months Ended
		Sep. 30, 2023	Dec. 31, 2022
Short-term Debt [Line Items]
Amount outstanding at period end		$ 0	$ 47,000,000
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
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	47,000,000
Average amount outstanding		0	18,000,000
Maximum amount outstanding		0	$ 123,000,000
Weighted average interest rate, computed on a daily basis			1.03%
Weighted average interest rate at period end			4.55%
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		0	0
Average amount outstanding		0	25,000,000
Maximum amount outstanding		$ 0	$ 81,000,000
Weighted average interest rate, computed on a daily basis			1.10%

[1] Expires in September 2027.