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Investments - Available-For-Sale Securities Disclosure (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | $ 7,983,271 | $ 7,880,697 |  |
Allowance for Credit Losses | (30,381) | (28,212) | $ (33,554) | $ (45,721)
Unrealized Gains | 41,627 | 50,076 |  |
Unrealized Losses | (411,013) | (403,364) |  |
Fair Value | 7,583,504 | 7,499,197 |  |
U.S. government and government agencies |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 161,335 | 223,157 |  |
Allowance for Credit Losses | 0 | 0 |  |
Unrealized Gains | 70 | 139 |  |
Unrealized Losses | (19,579) | (18,261) |  |
Fair Value | 141,826 | 205,035 |  |
Foreign government |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 10,703 | 11,140 |  |
Allowance for Credit Losses | (29) | (35) | (36) | (284)
Unrealized Gains | 0 | 0 |  |
Unrealized Losses | (1,445) | (1,302) |  |
Fair Value | 9,229 | 9,803 |  |
Obligations of states and political subdivisions |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 570,438 | 612,938 |  |
Allowance for Credit Losses | (695) | (669) | (737) | (1,024)
Unrealized Gains | 1,101 | 2,623 |  |
Unrealized Losses | (31,858) | (28,927) |  |
Fair Value | 538,986 | 585,965 |  |
Corporate securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 2,930,310 | 2,834,048 |  |
Allowance for Credit Losses | (15,442) | (12,999) | (16,756) | (30,330)
Unrealized Gains | 20,367 | 28,078 |  |
Unrealized Losses | (140,185) | (137,888) |  |
Fair Value | 2,795,050 | 2,711,239 |  |
Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS") |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 1,958,551 | 1,911,831 |  |
Allowance for Credit Losses | (2,627) | (2,854) | (3,895) | (2,375)
Unrealized Gains | 15,158 | 11,855 |  |
Unrealized Losses | (73,997) | (86,005) |  |
Fair Value | 1,897,085 | 1,834,827 |  |
Residential mortgage-backed securities ("RMBS") |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 1,623,948 | 1,568,960 |  |
Allowance for Credit Losses | (11,580) | (11,649) | (11,740) | (11,597)
Unrealized Gains | 3,828 | 6,023 |  |
Unrealized Losses | (104,237) | (85,851) |  |
Fair Value | 1,511,959 | 1,477,483 |  |
Commercial mortgage-backed securities ("CMBS") |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Cost/ Amortized Cost | 727,986 | 718,623 |  |
Allowance for Credit Losses | (8) | (6) | $ (390) | $ (111)
Unrealized Gains | 1,103 | 1,358 |  |
Unrealized Losses | (39,712) | (45,130) |  |
Fair Value | $ 689,369 | $ 674,845 |  |

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