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Significant Accounting Policies - Summary of Inventories (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Regulatory Assets [Abstract]    
Finished goods $ 47,613 $ 43,511
Work in process 35,455 32,839
Raw materials and supplies 87,128 70,099
Total Inventory $ 170,196 $ 146,449