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Fair Value Measurements (Details) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | $ 5,802,331 |  | $ 5,851,689
Equity securities | 15,038 |  | 14,762
Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 5,802,331 |  | 5,851,689
Equity securities | 15,038 |  | 14,762
Cash equivalents | 281,866 |  | 230,156
Total assets | 6,099,235 |  | 6,096,607
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 | 301,764 |  | 220,369
Equity securities | 15,038 |  | 14,762
Cash equivalents | 276,998 |  | 219,943
Total assets | 593,800 |  | 455,074
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 5,500,567 |  | 5,631,320
Equity securities | 0 |  | 0
Cash equivalents | 4,868 |  | 10,213
Total assets | 5,505,435 |  | 5,641,533
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 | 300 |  | (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 | 2,000 | $ 2,300 |
Sales of real estate assets acquired | 2,200 | $ 500 |
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 | 338,694 |  | 260,493
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 | 338,694 |  | 260,493
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 | 301,764 |  | 220,369
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 | 36,930 |  | 40,124
Obligations of U.S. states and political subdivisions |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 1,837,405 |  | 1,875,495
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 | 1,837,405 |  | 1,875,495
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 | 1,837,405 |  | 1,875,495
Corporate debt securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 2,678,441 |  | 2,757,381
Corporate debt securities | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 2,678,441 |  | 2,757,381
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 | 2,678,441 |  | 2,757,381
ABS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 186,155 |  | 156,517
ABS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 186,155 |  | 156,517
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 | 186,155 |  | 156,517
RMBS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 363,279 |  | 350,060
RMBS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 363,279 |  | 350,060
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 | 363,279 |  | 350,060
CMBS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 246,100 |  | 235,871
CMBS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 246,100 |  | 235,871
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 | 246,100 |  | 235,871
CLOs |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 124,678 |  | 200,059
CLOs | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 124,678 |  | 200,059
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 | 124,678 |  | 200,059
Foreign government debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 3,984 |  | 3,798
Foreign government debt | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 3,984 |  | 3,798
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 | 3,984 |  | 3,798
Commercial paper |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 23,595 |  | 12,015
Commercial paper | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 23,595 |  | 12,015
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 | $ 23,595 |  | $ 12,015

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