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Supplemental Cash Flow Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Supplemental disclosures:      
Interest paid $ 0 $ 0 $ 0
Income taxes paid, net 16,951 27,353 24,125
Non-cash investing and financing activities:      
Non-cash capital expenditures $ 832 $ 270 $ 83