v3.26.3
Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Vale on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Financial Assets:    
Fair Value $ 5,871,983 $ 6,104,085
Recurring    
Financial Assets:    
Total assets at fair value 5,871,983 6,104,085
Recurring | U.S. Treasury securities    
Financial Assets:    
Fair Value 317,844 369,712
Recurring | U.S. agency mortgage-backed securities    
Financial Assets:    
Fair Value 1,125,165 1,174,895
Recurring | Municipal debt securities    
Financial Assets:    
Fair Value 563,350 610,411
Recurring | Non-U.S. government securities    
Financial Assets:    
Fair Value 42,169 56,024
Recurring | Corporate debt securities:    
Financial Assets:    
Fair Value 1,933,530 1,980,080
Recurring | Residential and commercial mortgage securities    
Financial Assets:    
Fair Value 457,485 464,105
Recurring | Asset-backed securities    
Financial Assets:    
Fair Value 928,298 800,366
Recurring | Money market funds    
Financial Assets:    
Fair Value 504,142 648,492
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1)    
Financial Assets:    
Total assets at fair value 821,986 1,018,204
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. Treasury securities    
Financial Assets:    
Fair Value 317,844 369,712
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. agency mortgage-backed securities    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Municipal debt securities    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Non-U.S. government securities    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Corporate debt securities:    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Residential and commercial mortgage securities    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Asset-backed securities    
Financial Assets:    
Fair Value 0 0
Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Money market funds    
Financial Assets:    
Fair Value 504,142 648,492
Recurring | Significant Other Observable Inputs (Level 2)    
Financial Assets:    
Total assets at fair value 5,049,997 5,085,881
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency mortgage-backed securities    
Financial Assets:    
Fair Value 1,125,165 1,174,895
Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities    
Financial Assets:    
Fair Value 563,350 610,411
Recurring | Significant Other Observable Inputs (Level 2) | Non-U.S. government securities    
Financial Assets:    
Fair Value 42,169 56,024
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities:    
Financial Assets:    
Fair Value 1,933,530 1,980,080
Recurring | Significant Other Observable Inputs (Level 2) | Residential and commercial mortgage securities    
Financial Assets:    
Fair Value 457,485 464,105
Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities    
Financial Assets:    
Fair Value 928,298 800,366
Recurring | Significant Other Observable Inputs (Level 2) | Money market funds    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3)    
Financial Assets:    
Total assets at fair value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency mortgage-backed securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Municipal debt securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Non-U.S. government securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities:    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Residential and commercial mortgage securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities    
Financial Assets:    
Fair Value 0 0
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds    
Financial Assets:    
Fair Value $ 0 $ 0