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Marketable Securities - Available-For-Sale Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Available-For-Sale Securities    
Amortized Cost $ 31,375 $ 37,010
Gross Unrealized Gain 727 817
Gross Unrealized (Losses) 0 0
Estimated Fair Value 32,102 37,827
Tax-exempt municipal securities    
Available-For-Sale Securities    
Amortized Cost 18,493 19,924
Gross Unrealized Gain 418 365
Gross Unrealized (Losses) 0 0
Estimated Fair Value 18,911 20,289
Corporate debt securities    
Available-For-Sale Securities    
Amortized Cost 12,882 17,086
Gross Unrealized Gain 309 452
Gross Unrealized (Losses) 0 0
Estimated Fair Value $ 13,191 $ 17,538