Description of Business and Significant Accounting Policies - Investments in Marketable Securities and Inventories and Supplies (Details) - USD ($) $ in Thousands |
12 Months Ended | |
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Dec. 31, 2017 |
Dec. 31, 2016 |
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| Accounting Policies [Abstract] | ||
| Marketable securities, at fair value | $ 73,221 | $ 67,730 |
| Unrealized gains from marketable securities | $ 1,100 | |
| Amortization period of inventories and supplies | 24 months |
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- Definition Period of Amortization of Inventories and Supplies No definition available.
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- References No definition available.
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- Definition Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities during the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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