Fair Value Measurements - Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Financial Assets
Cash and cash equivalents	$ 388.8	$ 117.9
Financial Liabilities
Long-term debt	3,186.0	3,129.7
Long-term finance lease obligation	2.2	2.7
Unsecured Debt
Financial Liabilities
Unamortized debt issuance costs	4.3	4.8
Carrying Amount
Financial Assets
Cash and cash equivalents	388.8	117.9
Financial Liabilities
Long-term debt	3,238.0	3,236.5
Estimated Fair Value \| Level 1
Financial Assets
Cash and cash equivalents	388.8	117.9
Financial Liabilities
Long-term debt	0.0	0.0
Estimated Fair Value \| Level 2
Financial Assets
Cash and cash equivalents	0.0	0.0
Financial Liabilities
Long-term debt	3,186.0	3,129.7
Estimated Fair Value \| Level 3
Financial Assets
Cash and cash equivalents	0.0	0.0
Financial Liabilities
Long-term debt	$ 0.0	$ 0.0

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Present value of lessee's discounted obligation for lease payments from finance lease, classified as noncurrent.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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