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Investments - Schedule of Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Dec. 31, 2021
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U.S. government |  |
Fair value |  |
Less than 12 months | $ 142,944 | $ 2,942
Total Fair Value | 142,944 | 2,942
Amortized Cost |  |
Amortized cost, less than 12 months | 144,854 | 2,986
Total Cost or Amortized Cost | 144,854 | 2,986
Unrealized Loss |  |
Less than 12 months | (1,910) | (44)
Total Unrealized Loss | (1,910) | (44)
U.S. Agency |  |
Fair value |  |
Less than 12 months | 15,788 | 1,498
Total Fair Value | 15,788 | 1,498
Amortized Cost |  |
Amortized cost, less than 12 months | 16,625 | 1,515
Total Cost or Amortized Cost | 16,625 | 1,515
Unrealized Loss |  |
Less than 12 months | (837) | (17)
Total Unrealized Loss | (837) | (17)
Non-U.S. government & agency |  |
Fair value |  |
Less than 12 months | 3,731 | 4,346
12 months and greater | 2,337 |
Total Fair Value | 6,068 | 4,346
Amortized Cost |  |
Amortized cost, less than 12 months | 3,797 | 4,500
12 months and greater | 3,000 |
Total Cost or Amortized Cost | 6,797 | 4,500
Unrealized Loss |  |
Less than 12 months | (66) | (154)
12 months and greater | (663) |
Total Unrealized Loss | (729) | (154)
Agency MBS |  |
Fair value |  |
Less than 12 months | 221,563 | 102,145
12 months and greater | 68,070 | 62,669
Total Fair Value | 289,633 | 164,814
Amortized Cost |  |
Amortized cost, less than 12 months | 236,671 | 104,336
12 months and greater | 79,471 | 65,429
Total Cost or Amortized Cost | 316,142 | 169,765
Unrealized Loss |  |
Less than 12 months | (15,108) | (2,191)
12 months and greater | (11,401) | (2,760)
Total Unrealized Loss | (26,509) | (4,951)
ABS/CMBS/MBS |  |
Fair value |  |
Less than 12 months | 214,266 | 150,997
12 months and greater | 22,312 | 3,935
Total Fair Value | 236,578 | 154,932
Amortized Cost |  |
Amortized cost, less than 12 months | 236,298 | 153,235
12 months and greater | 25,635 | 4,036
Total Cost or Amortized Cost | 261,933 | 157,271
Unrealized Loss |  |
Less than 12 months | (22,032) | (2,238)
12 months and greater | (3,323) | (101)
Total Unrealized Loss | (25,355) | (2,339)
Corporate Debt |  |
Fair value |  |
Less than 12 months | 762,770 | 217,791
12 months and greater | 78,901 | 53,818
Total Fair Value | 841,671 | 271,609
Amortized Cost |  |
Amortized cost, less than 12 months | 821,145 | 221,010
12 months and greater | 90,669 | 55,704
Total Cost or Amortized Cost | 911,814 | 276,714
Unrealized Loss |  |
Less than 12 months | (58,375) | (3,219)
12 months and greater | (11,768) | (1,886)
Total Unrealized Loss | (70,143) | (5,105)
Municipal |  |
Fair value |  |
Less than 12 months | 387,833 | 162,998
12 months and greater | 30,986 | 15,037
Total Fair Value | 418,819 | 178,035
Amortized Cost |  |
Amortized cost, less than 12 months | 454,930 | 166,602
12 months and greater | 41,056 | 15,714
Total Cost or Amortized Cost | 495,986 | 182,316
Unrealized Loss |  |
Less than 12 months | (67,097) | (3,604)
12 months and greater | (10,070) | (677)
Total Unrealized Loss | (77,167) | (4,281)
Fixed Income Securities |  |
Fair value |  |
Less than 12 months | 1,748,895 | 642,717
12 months and greater | 202,606 | 135,459
Total Fair Value | 1,951,501 | 778,176
Amortized Cost |  |
Amortized cost, less than 12 months | 1,914,320 | 654,184
12 months and greater | 239,831 | 140,883
Total Cost or Amortized Cost | 2,154,151 | 795,067
Unrealized Loss |  |
Less than 12 months | (165,425) | (11,467)
12 months and greater | (37,225) | (5,424)
Total Unrealized Loss | $ (202,650) | $ (16,891)

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