Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - PPE (Details)

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Summary of Significant Accounting Policies - PPE (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, plant and equipment      
Depreciation expense $ 7,183 $ 6,146 $ 4,919
Repairs and maintenance expenses $ 5,222 $ 5,060 $ 5,272
Land improvements and buildings | Minimum      
Property, plant and equipment      
Estimated useful lives 15 years    
Land improvements and buildings | Maximum      
Property, plant and equipment      
Estimated useful lives 40 years    
Leasehold Improvements | Minimum      
Property, plant and equipment      
Estimated useful lives 12 years    
Machinery and equipment | Minimum      
Property, plant and equipment      
Estimated useful lives 3 years    
Machinery and equipment | Maximum      
Property, plant and equipment      
Estimated useful lives 20 years    
Furniture and fixtures | Minimum      
Property, plant and equipment      
Estimated useful lives 3 years    
Furniture and fixtures | Maximum      
Property, plant and equipment      
Estimated useful lives 12 years    
Mobile equipment and other | Minimum      
Property, plant and equipment      
Estimated useful lives 3 years    
Mobile equipment and other | Maximum      
Property, plant and equipment      
Estimated useful lives 10 years