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Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Marketable Securities [Line Items]    
Amortized cost $ 4,429 $ 4,521
Gross unrealized holding gains 28 26
Gross unrealized holding losses (20) (4)
Estimated fair value 4,437 4,543
Corporate bonds and notes [Member]    
Marketable Securities [Line Items]    
Amortized cost 1,709 1,814
Gross unrealized holding gains 14 21
Gross unrealized holding losses (20) (3)
Estimated fair value 1,703 1,832
Municipal bonds [Member]    
Marketable Securities [Line Items]    
Amortized cost 2,720 2,707
Gross unrealized holding gains 14 5
Gross unrealized holding losses (1)
Estimated fair value $ 2,734 $ 2,711