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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Cash and Investments in Marketable Securities [Line Items] |  |  |
Sales of investments | $ 0 | $ 11,963,000 | $ 37,549,000
Level 1 to level 2 transfers | 0 | 0 |
Level 2 to level 1 transfers | 0 | 0 |
Letter of credit | $ 6,800,000 | $ 6,600,000 |
Maximum |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |
Long term investment maturity period | 2 years |  |

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