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Marketable Investments - Summary of Company's Marketable Investments (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 54,600 | $ 54,521
Gross Unrealized Losses | (342) | (188)
Market Value | 54,258 | 54,333
Federal Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,800 | 1,800
Gross Unrealized Losses | (6) | (7)
Market Value | 1,794 | 1,793
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 52,800 | 52,721
Gross Unrealized Losses | (336) | (181)
Market Value | $ 52,464 | $ 52,540

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