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Investments - Schedule of Amortized Cost and Fair Value of Investments Available for Sale by Contractual Maturity (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Amortized Cost    
Amortized Cost $ 5,502,929 $ 5,184,856
Fair Value    
Fair Value 4,997,688 4,741,625
U.S. Treasury securities    
Amortized Cost    
Due in 1 year 526,416  
Due after 1 but within 5 years 305,939  
Due after 5 but within 10 years 62,373  
Due after 10 years 14,410  
Subtotal 909,138  
Amortized Cost 909,138 584,173
Fair Value    
Due in 1 year 524,639  
Due after 1 but within 5 years 286,589  
Due after 5 but within 10 years 55,799  
Due after 10 years 12,158  
Subtotal 879,185  
U.S. agency securities    
Amortized Cost    
Due in 1 year 7,198  
Subtotal 7,198  
Amortized Cost 7,198 49,059
Fair Value    
Due in 1 year 7,180  
Subtotal 7,180  
Municipal debt securities    
Amortized Cost    
Due in 1 year 2,388  
Due after 1 but within 5 years 101,186  
Due after 5 but within 10 years 168,182  
Due after 10 years 365,780  
Subtotal 637,536  
Amortized Cost 637,536 661,934
Fair Value    
Due in 1 year 2,347  
Due after 1 but within 5 years 96,172  
Due after 5 but within 10 years 153,099  
Due after 10 years 310,892  
Subtotal 562,510  
Non-U.S. government securities:    
Amortized Cost    
Due after 1 but within 5 years 32,135  
Due after 5 but within 10 years 5,064  
Due after 10 years 20,208  
Subtotal 57,407  
Amortized Cost 57,407 69,651
Fair Value    
Due after 1 but within 5 years 30,496  
Due after 5 but within 10 years 3,945  
Due after 10 years 14,130  
Subtotal 48,571  
Corporate debt securities    
Amortized Cost    
Due in 1 year 194,084  
Due after 1 but within 5 years 569,356  
Due after 5 but within 10 years 483,787  
Due after 10 years 181,646  
Subtotal 1,428,873  
Amortized Cost 1,428,873 1,546,513
Fair Value    
Due in 1 year 190,994  
Due after 1 but within 5 years 534,150  
Due after 5 but within 10 years 415,847  
Due after 10 years 139,442  
Subtotal 1,280,433  
U.S. agency mortgage-backed securities    
Amortized Cost    
Without single maturity date 898,677  
Amortized Cost 898,677 898,675
Fair Value    
Without single maturity date 750,938  
Residential and commercial mortgage securities    
Amortized Cost    
Without single maturity date 563,358  
Amortized Cost 563,358 577,915
Fair Value    
Without single maturity date 493,693  
Asset-backed securities    
Amortized Cost    
Without single maturity date 640,382  
Amortized Cost 640,382 660,345
Fair Value    
Without single maturity date 614,818  
Money market funds    
Amortized Cost    
Without single maturity date 360,360  
Amortized Cost 360,360 $ 136,591
Fair Value    
Without single maturity date $ 360,360