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Accounts Receivable - Summary of Activity in Allowances for Doubtful Accounts, Returns and Discounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]      
Balance at Beginning of Period $ 4,831 $ 4,340 $ 7,057
Charges to Revenue 4,030 1,488 1,826
Charges (Credits) to Expense 881 1,100 208
Additions to (Deductions from) Reserves (5,111) (2,097) (4,751)
Balance at End of Period $ 4,631 $ 4,831 $ 4,340