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Investments - Marketable Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost, Current $ 751 $ 2,215
Municipal bonds    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost, Current 751 2,215
Amortized Cost, Due from one through five years 6,705 7,420
Amortized Cost, Due from six through ten years 2,734 3,057
Amortized Cost, Due from eleven through twenty years 1,199 2,323
Amortized Cost, Total 11,389 15,015
Market Value, Current 755 2,249
Market Value, Due from one through five years 7,050 7,830
Market Value, Due from six through ten years 3,188 3,608
Market Value, Due from eleven through twenty years 1,300 2,547
Market Value, Total $ 12,293 $ 16,234