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Inventories - Schedule of Inventories (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Inventory Disclosure [Abstract]      
Raw materials $ 275 $ 298 $ 346
Work-in-process 682 721 214
Finished goods 323 229 100
Total inventories $ 1,280 $ 1,248 $ 660