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Fair Value Measurements (Details) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | $ 5,603,038 |  | $ 5,409,698
Equity securities | 14,384 |  | 14,140
Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | 14,384 |  | 14,140
Cash equivalents | 313,229 |  | 328,756
Total assets | 5,930,651 |  | 5,752,594
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] |  |  |
Equity securities | 14,384 |  | 14,140
Cash equivalents | 296,783 |  | 324,129
Total assets | 407,972 |  | 455,166
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | 0 |  | 0
Cash equivalents | 16,446 |  | 4,627
Total assets | 5,522,679 |  | 5,297,428
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 | 100 |  | 2,500
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 | 100 | $ 1,500 |
Sales of real estate assets acquired | 1,200 | $ 2,400 |
Total fixed income securities (1) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 5,603,038 |  | 5,409,698
Total fixed income securities (1) | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 5,603,038 |  | 5,409,698
Total fixed income securities (1) | 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 | 96,805 |  | 116,897
Total fixed income securities (1) | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 5,506,233 |  | 5,292,801
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 | 176,376 |  | 135,900
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 | 176,376 |  | 135,900
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 | 96,805 |  | 116,897
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 | 79,571 |  | 19,003
Obligations of U.S. states and political subdivisions |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 1,998,178 |  | 2,149,054
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 | 1,998,178 |  | 2,149,054
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 | 1,998,178 |  | 2,149,054
Corporate debt securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 2,365,402 |  | 2,221,141
Corporate debt securities | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 2,365,402 |  | 2,221,141
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 | 2,365,402 |  | 2,221,141
ABS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 121,954 |  | 120,687
ABS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 121,954 |  | 120,687
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 | 121,954 |  | 120,687
RMBS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 236,571 |  | 198,009
RMBS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 236,571 |  | 198,009
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 | 236,571 |  | 198,009
CMBS |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 279,572 |  | 237,216
CMBS | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 279,572 |  | 237,216
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 | 279,572 |  | 237,216
CLOs |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 330,451 |  | 329,832
CLOs | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 330,451 |  | 329,832
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 | 330,451 |  | 329,832
Foreign government debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 3,743 |  | 3,787
Foreign government debt | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 3,743 |  | 3,787
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 | 3,743 |  | 3,787
Commercial paper |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 90,791 |  | 14,072
Commercial paper | Fair Value, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value | 90,791 |  | 14,072
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 | $ 90,791 |  | $ 14,072

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