Summary of Significant Accounting Policies - Chargebacks, Rebates, Administrative Fees and Other, and Sales Returns (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contract with customer, payment terms			90 days
Restricted cash	$ 799,000	$ 803,000	$ 799,000	$ 803,000	$ 900,000
Extended chargeback period			84 days
Chargeback provision	$ 141,943,000	$ 188,884,000	$ 540,055,000	635,329,000
Sales chargebacks as percentage of gross sales	37.40%	43.30%
Gross sales	$ 379,022,000	$ 436,700,000	$ 1,303,671,000	1,465,052,000
Chargeback reserves	2.49%		2.49%
Potential increase in chargeback expense based on sensitivity analysis	$ 400,000		$ 400,000
Potential decrease in chargeback expense based on sensitivity analysis	900,000		900,000
Rebates, administrative and other fees	$ 46,212,000	$ 66,843,000	$ 188,445,000	234,217,000
Rebates, administrative and other fees as a percentage of gross sales	12.20%	15.30%
Rebates, administrative and other fees	2.49%		2.49%
Potential increase in rebates, administrative and other fees expense based on sensitivity analysis	$ 25,700,000		$ 25,700,000
Potential decrease in rebates, administrative and other fees expense based on sensitivity analysis	100,000		100,000
Sales returns	$ 5,900,000	$ 4,805,000	$ 23,273,000	$ 18,059,000
Change in ratio of direct to indirect sales for sales return reserve used in sensitivity analysis	1.60%	1.10%
Returns, change in average return time			6 months
Potential increase in sales returns expense based on sensitivity analysis	$ 400,000		$ 400,000
Potential decrease in sales returns expense based on sensitivity analysis	$ 500,000		$ 500,000
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