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Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Marketable Securities [Line Items]    
Amortized cost $ 3,933 $ 4,521
Gross unrealized holding gains 46 26
Gross unrealized holding losses (8) (4)
Estimated fair value 3,971 4,543
Corporate bonds and notes [Member]    
Marketable Securities [Line Items]    
Amortized cost 1,916 1,814
Gross unrealized holding gains 37 21
Gross unrealized holding losses (8) (3)
Estimated fair value 1,945 1,832
Municipal bonds [Member]    
Marketable Securities [Line Items]    
Amortized cost 2,017 2,707
Gross unrealized holding gains 9 5
Gross unrealized holding losses (1)
Estimated fair value $ 2,026 $ 2,711