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Investments - Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands
Dec. 31, 2022
Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 2,352,747 $ 2,078,773
Gross Unrealized Gains 1,334 30,353
Gross Unrealized Losses (254,692) (23,195)
Fair Value 2,099,389 2,085,931
U.S. Treasury securities and obligations of U.S. government agencies    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 82,301 29,443
Gross Unrealized Gains 0 981
Gross Unrealized Losses (2,369) 0
Fair Value 79,932 30,424
Municipal debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 563,972 553,793
Gross Unrealized Gains 0 5,689
Gross Unrealized Losses (80,796) (5,404)
Fair Value 483,176 554,078
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 1,457,589 1,388,204
Gross Unrealized Gains 1,149 22,990
Gross Unrealized Losses (165,096) (17,364)
Fair Value 1,293,642 1,393,830
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 74,762 96,324
Gross Unrealized Gains 0 684
Gross Unrealized Losses (6,204) (427)
Fair Value 68,558 96,581
Total bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,178,624 2,067,764
Gross Unrealized Gains 1,149 30,344
Gross Unrealized Losses (254,465) (23,195)
Fair Value 1,925,308 2,074,913
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 174,123 11,009
Gross Unrealized Gains 185 9
Gross Unrealized Losses (227) 0
Fair Value $ 174,081 $ 11,018