Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2024	Dec. 31, 2023
Short-term Debt [Line Items]
Amount outstanding at period end		$ 0	$ 60,000,000
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	60,000,000
Average amount outstanding		41,000,000	15,000,000
Maximum amount outstanding		$ 99,000,000	$ 93,000,000
Weighted average interest rate, computed on a daily basis		5.49%	4.85%
Weighted average interest rate at period end			5.50%
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		19,000,000	0
Average amount outstanding		12,000,000	6,000,000
Maximum amount outstanding		$ 54,000,000	$ 43,000,000
Weighted average interest rate, computed on a daily basis		5.33%	4.98%
Weighted average interest rate at period end		5.33%

[1] Expires in September 2027.