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Basis of Presentation and Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for doubtful accounts receivable [Rollforward]      
Beginning balance $ 2,574 $ 686 $ 172
Add: bad debt expense 4,289 1,890 542
Less: write-offs, net of recoveries (4,606) (2) (28)
Ending balance $ 2,257 $ 2,574 $ 686