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Commitments and Contingencies - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies      
Rent expense $ 8,200,000 $ 4,800,000 $ 2,200,000
Indemnifications      
Commitments and Contingencies      
Recorded obligation $ 0 $ 0