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Fair Value Measurements (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value $ 5,703,121   $ 5,791,231
Equity securities 15,191   15,322
Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 5,703,121   5,791,231
Equity securities 15,191   15,322
Cash equivalents 245,491   371,219
Total assets 5,963,803   6,177,772
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 198,760   241,056
Equity securities 15,191   15,322
Cash equivalents 224,114   363,423
Total assets 438,065   619,801
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 5,504,361   5,550,175
Equity securities 0   0
Cash equivalents 21,377   7,796
Total assets 5,525,738   5,557,971
Significant Unobservable Inputs (Level 3)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Embedded derivative related to Home Re transactions (600)   (400)
Significant Unobservable Inputs (Level 3) | Real Estate Acquired      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Purchases of real estate assets acquired 1,200 $ 700  
Sales of real estate assets acquired 1,700 $ 1,100  
U.S. Treasury securities and obligations of U.S. government corporations and agencies      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 233,630   277,812
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 233,630   277,812
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 198,760   241,056
U.S. Treasury securities and obligations of U.S. government corporations and agencies | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 34,870   36,756
Obligations of U.S. states and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 1,772,408   1,838,764
Obligations of U.S. states and political subdivisions | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 1,772,408   1,838,764
Obligations of U.S. states and political subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
Obligations of U.S. states and political subdivisions | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 1,772,408   1,838,764
Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 2,680,137   2,691,272
Corporate debt securities | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 2,680,137   2,691,272
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 2,680,137   2,691,272
ABS      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 262,641   226,561
ABS | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 262,641   226,561
ABS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
ABS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 262,641   226,561
RMBS      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 350,710   358,024
RMBS | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 350,710   358,024
RMBS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
RMBS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 350,710   358,024
CMBS      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 258,917   238,783
CMBS | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 258,917   238,783
CMBS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
CMBS | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 258,917   238,783
CLOs      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 131,221   99,287
CLOs | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 131,221   99,287
CLOs | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
CLOs | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 131,221   99,287
Foreign government debt      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 4,059   4,067
Foreign government debt | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 4,059   4,067
Foreign government debt | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
Foreign government debt | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 4,059   4,067
Commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 9,398   56,661
Commercial paper | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 9,398   56,661
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value 0   0
Commercial paper | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value $ 9,398   $ 56,661