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Investments (Available-For-Sale Securities by Contractual Maturity) (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost $ 3,742,234 $ 3,852,411
Total Fair Value 3,355,312 3,472,472
U.S. Treasury obligations    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 261,067 243,999
Due in one year or less 35,570  
Due after one year through five years 166,215  
Due after five years through ten years 33,643  
Due after ten years 2,400  
Total Fair Value 237,828 221,608
U.S. Government-sponsored enterprise obligations    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 20,354 21,562
Due in one year or less 5,186  
Due after one year through five years 13,199  
Due after five years through ten years 486  
Due after ten years 0  
Total Fair Value 18,871 19,934
State and municipal bonds    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 480,277 483,584
Due in one year or less 36,817  
Due after one year through five years 150,101  
Due after five years through ten years 140,152  
Due after ten years 109,198  
Total Fair Value 436,268 439,450
Corporate debt    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1,864,148 1,980,579
Due in one year or less 216,641  
Due after one year through five years 830,290  
Due after five years through ten years 534,892  
Due after ten years 82,155  
Total Fair Value 1,663,978 1,781,452
Residential mortgage-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 470,066 450,870
Total Fair Value 396,947 389,540
Agency commercial mortgage-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 9,768 10,576
Total Fair Value 8,652 9,704
Other commercial mortgage-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 210,104 217,021
Total Fair Value 185,809 194,090
Other asset-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 426,450 444,220
Total Fair Value $ 406,959 $ 416,694