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Investments - Schedule of Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 3,078,294 $ 2,876,343
Gross Unrealized, Gains 32,775 5,820
Gross Unrealized, Losses (95,509) (158,622)
Fair Value 3,015,560 2,723,541
U.S. Treasury securities and obligations of U.S. government agencies    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 100,950 115,342
Gross Unrealized, Gains 2,710 1,207
Gross Unrealized, Losses (165) (489)
Fair Value 103,495 116,060
Municipal debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 658,725 684,523
Gross Unrealized, Gains 2,603 589
Gross Unrealized, Losses (30,964) (49,867)
Fair Value 630,364 635,245
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,148,007 1,949,800
Gross Unrealized, Gains 27,454 3,981
Gross Unrealized, Losses (61,582) (106,141)
Fair Value 2,113,879 1,847,640
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 52,559 44,104
Gross Unrealized, Gains 0 1
Gross Unrealized, Losses (2,796) (2,125)
Fair Value 49,763 41,980
Total bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,960,241 2,793,769
Gross Unrealized, Gains 32,767 5,778
Gross Unrealized, Losses (95,507) (158,622)
Fair Value 2,897,501 2,640,925
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 118,053 82,574
Gross Unrealized, Gains 8 42
Gross Unrealized, Losses (2) 0
Fair Value $ 118,059 $ 82,616