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Business Combinations - Summary of Business Acquisitions (Parenthetical) (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
shares
Business Acquisition [Line Items]  
Purchase consideration, value of shares issued $ 10,859
Alpha [Member]  
Business Acquisition [Line Items]  
Business combination contingencies amount recognized $ 21,700
Purchase consideration, number of shares issued | shares 282,577
Purchase consideration, value of shares issued $ 10,859