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Reclassifications Out of Accumulated Other Comprehensive Income to Statements of Operations (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Reclassifications, tax | $ 667 | $ 235 | $ 92
Unrealized (Losses) Gains on Marketable Securities |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Reclassifications, tax | 665 | 192 | 21
Amortization of Prior Service Credit |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |
Reclassifications, tax | $ 2 | $ 43 | $ 71

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