Borrowings and Other Financing Instruments - Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Short-term Debt [Line Items]
Short-term debt	$ 268,000,000	$ 320,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Short-term debt	250,000,000	51,000,000
Average amount outstanding	91,000,000	23,000,000
Maximum amount outstanding	$ 250,000,000	$ 250,000,000
Weighted average interest rate, computed on a daily basis (percentage)	5.33%	5.31%
Short-term Debt, Weighted Average Interest Rate, at Point in Time	5.33%	5.34%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000
Short-term debt	268,000,000	320,000,000
Average amount outstanding	270,000,000	124,000,000
Maximum amount outstanding	$ 492,000,000	$ 454,000,000
Weighted average interest rate, computed on a daily basis (percentage)	5.58%	5.17%
Short-term Debt, Weighted Average Interest Rate, at Point in Time	5.49%	5.56%