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Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Detail)
$ in Millions
Dec. 31, 2023
USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost $ 8,717.2
Market value 7,971.9
Amount at which shown in the balance sheet 7,971.3
Short-term Investments  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 55.8
Market value 55.6
Amount at which shown in the balance sheet 55.8
U.S. Treasury bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 203.3
Market value 199.3
Amount at which shown in the balance sheet 199.3
Municipal bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 1,373.7
Market value 1,245.8
Amount at which shown in the balance sheet 1,245.8
Foreign government bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 228.4
Market value 219.3
Amount at which shown in the balance sheet 219.3
Governmental agency bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 207.7
Market value 195.4
Amount at which shown in the balance sheet 195.4
Governmental agency mortgage-backed securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 4,396.2
Market value 3,875.7
Amount at which shown in the balance sheet 3,875.7
U.S. corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 1,007.0
Market value 958.4
Amount at which shown in the balance sheet 958.4
Foreign corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 478.9
Market value 463.6
Amount at which shown in the balance sheet 463.6
Debt Securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 7,895.2
Market value 7,157.5
Amount at which shown in the balance sheet 7,157.5
Equity Securities, Investment Summary  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 743.8 [1]
Market value 735.6 [1]
Amount at which shown in the balance sheet 735.6 [1]
Notes Receivable  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 22.4
Market value 23.2
Amount at which shown in the balance sheet $ 22.4
[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.