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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

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