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Summary of Gross Realized Gains and Losses from Sale of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Gross realized gains from sale of available-for-sale marketable securities |  | $ 6 | $ 4 | $ 6
Gross realized losses from sale of available-for-sale marketable securities | $ (2) | $ (8) | $ (2) | $ (46)

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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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Balance Type: | debit
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- References No definition available. + Details
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