Summary of Significant Accounting Policies - Chargebacks, Rebates, Administrative Fees and Other, and Sales Returns (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contract with customer, payment terms	90 days
Restricted cash	$ 785	$ 2,372	$ 900
Extended chargeback period	84 days
Chargeback provision	$ 205,394	$ 223,963
Sales chargebacks as percentage of gross sales	44.80%	43.00%
Gross sales	$ 458,641	$ 520,533
Chargeback reserves	5.60%
Potential increase in chargeback expense based on sensitivity analysis	$ 700
Potential decrease in chargeback expense based on sensitivity analysis	2,900
Rebates, administrative and other fees	$ 64,264	$ 92,279
Rebates, administrative and other fees as a percentage of gross sales	14.00%	17.70%
Rebates, administrative and other fees	5.60%
Potential increase in rebates, administrative and other fees expense based on sensitivity analysis	$ 0
Potential decrease in rebates, administrative and other fees expense based on sensitivity analysis	400
Sales returns	$ 11,902	$ 7,121
Change in ratio of direct to indirect sales for sales return reserve used in sensitivity analysis	2.60%	1.40%
Returns, change in average return time	3 months 18 days
Potential increase in sales returns expense based on sensitivity analysis	$ 300
Potential decrease in sales returns expense based on sensitivity analysis	$ 400
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Standard chargeback period	28 days
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Standard chargeback period	42 days