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Supplemental Balance Sheet, Income Statement and Cash Flow Information - Investment income (loss) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Investment income (loss) |  |  |
Interest and dividend income | $ 4,705 | $ 8,424 | $ 4,957
Gains (losses) on marketable securities | (3,267) | 1,794 | (14,456)
Investment income (loss) | $ 1,438 | $ 10,218 | $ (9,499)

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