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Property, Plant and Equipment - Depreciation and Amortization (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Depreciation and amortization $ 122,102 $ 114,215 $ 108,962
Services      
Property, Plant and Equipment [Line Items]      
Depreciation and amortization 102,212 101,260 95,825
Products      
Property, Plant and Equipment [Line Items]      
Depreciation and amortization 4,971 4,254 4,233
General and administrative expenses      
Property, Plant and Equipment [Line Items]      
Depreciation and amortization $ 14,919 $ 8,701 $ 8,904