Summary of Change in Reserves for Discounts and Allowances (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	$ 8,587
Provision	34,605
Payments/credits	(20,039)
Balance, as of September 30, 2018	23,153
Chargebacks [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	995
Provision	9,308
Payments/credits	(9,502)
Balance, as of September 30, 2018	801
Rebates [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	6,959
Provision	19,298
Payments/credits	(6,479)
Balance, as of September 30, 2018	19,778
Prompt Pay [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	169
Provision	1,804
Payments/credits	(1,526)
Balance, as of September 30, 2018	447
Other [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	464
Provision	4,195
Payments/credits	(2,532)
Balance, as of September 30, 2018	$ 2,127