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Marketable Equity Securities - Marketable Equity Securities and Adjustments to Fair Value (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]    
Marketable equity securities at cost $ 1,714 $ 1,714
Cumulative unrealized loss on marketable equity securities (455) (129)
Marketable equity securities at fair value $ 1,259 $ 1,585