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Marketable Securities - Additional Information (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract] |  |
Number of investment-grade securities in unrealized loss position over 12 months | $ 0 |
Interest Receivable | $ 100 | $ 300

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