Summary of Significant Accounting Policies - Chargebacks, Rebates, Administrative Fees and Other, and Sales Returns (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contract with customer, payment terms			90 days
Restricted cash	$ 803	$ 837	$ 803	$ 837	$ 900
Extended chargeback period			84 days
Chargeback provision	$ 192,717	$ 222,482	$ 398,111	446,445
Sales chargebacks as percentage of gross sales	41.40%	43.80%
Gross sales	$ 466,009	$ 507,819	$ 924,649	1,028,352
Chargeback reserves	2.02%		2.02%
Potential increase in chargeback expense based on sensitivity analysis	$ 300		$ 300
Potential decrease in chargeback expense based on sensitivity analysis	800		800
Rebates, administrative and other fees	$ 77,969	$ 75,094	$ 142,233	167,374
Rebates, administrative and other fees as a percentage of gross sales	16.70%	14.80%
Rebates, administrative and other fees	2.02%		2.02%
Potential increase in rebates, administrative and other fees expense based on sensitivity analysis	$ 100		$ 100
Potential decrease in rebates, administrative and other fees expense based on sensitivity analysis	100		100
Sales returns	$ 5,471	$ 6,133	$ 17,373	$ 13,254
Change in ratio of direct to indirect sales for sales return reserve used in sensitivity analysis	1.20%	1.20%
Returns, change in average return time			3 months 18 days
Potential increase in sales returns expense based on sensitivity analysis	$ 400		$ 400
Potential decrease in sales returns expense based on sensitivity analysis	$ 100		$ 100
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