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Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Detail)
$ in Millions
Dec. 31, 2024
USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost $ 8,564.5
Market value 8,077.1
Amount at which shown in the balance sheet 8,077.0
Short-term Investments  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 85.4
Market value 85.3
Amount at which shown in the balance sheet 85.4
U.S. Treasury bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 180.7
Market value 175.6
Amount at which shown in the balance sheet 175.6
Municipal bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 844.9
Market value 776.5
Amount at which shown in the balance sheet 776.5
Foreign government bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 217.1
Market value 211.6
Amount at which shown in the balance sheet 211.6
Governmental agency bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 203.8
Market value 189.8
Amount at which shown in the balance sheet 189.8
Governmental agency mortgage-backed securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 4,844.4
Market value 4,502.3
Amount at which shown in the balance sheet 4,502.3
U.S. corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 948.4
Market value 925.6
Amount at which shown in the balance sheet 925.6
Foreign corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 491.6
Market value 484.5
Amount at which shown in the balance sheet 484.5
Debt Securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 7,730.9
Market value 7,265.9
Amount at which shown in the balance sheet 7,265.9
Equity Securities, Investment Summary  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 713.8 [1]
Market value 691.3 [1]
Amount at which shown in the balance sheet 691.3 [1]
Notes Receivable  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 34.4
Market value 34.6
Amount at which shown in the balance sheet $ 34.4
[1] Included in equity securities are non-marketable equity securities and equity method investments, at carrying amount. Estimates of fair value for these investments could not be made without incurring excessive costs.