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Supplemental Cash Flow Information - Supplemental Cash Flow Disclosure (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]      
Interest paid $ 18,956 $ 15,620 $ 13,436
Income taxes paid $ 1,756 1,100 $ 14,984
Computer software      
Finite-Lived Intangible Assets [Line Items]      
Intangible software assets acquired   $ 7,100