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Investments - Schedule of Available-For-Sale Securities Disclosure (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Mar. 31, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost $ 8,871,026 $ 8,476,078    
Allowance for Credit Losses (30,175) (31,948) $ (30,381) $ (28,212)
Unrealized Gains 71,786 53,285    
Unrealized Losses (307,462) (370,081)    
Fair Value 8,605,175 8,127,334    
U.S. government and government agencies        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 151,030 139,906    
Allowance for Credit Losses 0 0    
Unrealized Gains 44 2    
Unrealized Losses (16,599) (19,753)    
Fair Value 134,475 120,155    
Foreign government        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 11,535 10,656    
Allowance for Credit Losses (19) (21) (29) (35)
Unrealized Gains 22 0    
Unrealized Losses (1,150) (1,333)    
Fair Value 10,388 9,302    
Obligations of states and political subdivisions        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 462,904 483,609    
Allowance for Credit Losses (419) (570) (695) (669)
Unrealized Gains 1,109 550    
Unrealized Losses (30,644) (32,359)    
Fair Value 432,950 451,230    
Corporate securities        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 3,312,258 3,181,046    
Allowance for Credit Losses (12,616) (14,924) (15,442) (12,999)
Unrealized Gains 38,459 25,259    
Unrealized Losses (100,880) (123,201)    
Fair Value 3,237,221 3,068,180    
Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS")        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 2,228,290 2,065,611    
Allowance for Credit Losses (5,499) (4,889) (2,627) (2,854)
Unrealized Gains 21,067 22,116    
Unrealized Losses (46,673) (49,689)    
Fair Value 2,197,185 2,033,149    
Residential mortgage-backed securities ("RMBS")        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 1,943,238 1,812,744    
Allowance for Credit Losses (11,342) (11,544) (11,580) (11,649)
Unrealized Gains 8,404 3,880    
Unrealized Losses (88,221) (112,722)    
Fair Value 1,852,079 1,692,358    
Commercial mortgage-backed securities ("CMBS")        
Debt Securities, Available-for-sale [Line Items]        
Cost/ Amortized Cost 761,771 782,506    
Allowance for Credit Losses (280) 0 $ (8) $ (6)
Unrealized Gains 2,681 1,478    
Unrealized Losses (23,295) (31,024)    
Fair Value $ 740,877 $ 752,960