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Description of Business and Significant Accounting Policies - Investments in Marketable Securities and Inventories and Supplies (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]    
Marketable securities, at fair value $ 67,730 $ 69,496
Unrealized gains from marketable securities $ 900  
Amortization period of inventories and supplies 24 months