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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounting Policies [Abstract]
Payment terms period			90 days
Restricted cash	$ 837	$ 2,381	$ 837	$ 2,381	$ 1,800
Chargeback provision	$ 222,482	$ 237,275	446,445	517,437
Sales chargebacks as percentage of gross sales	43.80%	41.80%
Gross sales	$ 507,819	$ 567,112	$ 1,028,352	1,247,647
Change in ratio of direct to indirect sales for chargeback reserves used in sensitivity analysis	4.80%		4.80%
Potential increase in chargeback expense based on sensitivity analysis	$ 500		$ 500
Potential decrease in chargeback expense based on sensitivity analysis	2,600		2,600
Rebates, administrative and other fees	$ 75,094	$ 109,760	$ 167,374	234,138
Rebates, administrative and other fees as a percentage of gross sales	14.80%	19.40%
Change in ratio of direct to indirect sales for rebates, administrative and other fees used in sensitivity analysis	4.80%		4.80%
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	500		500
Sales returns	$ 6,133	$ 8,116	$ 13,254	$ 16,533
Change in ratio of direct to indirect sales for sales return reserve used in sensitivity analysis	1.20%	1.40%
Change in lag time to process returns for sales returns reserve used in sensitivity analysis			1 month
Potential decrease in sales returns expense based on sensitivity analysis	$ 1,500		$ 1,500
Potential increase in sales returns expense based on sensitivity analysis	$ 1,600		$ 1,600
Accounts, Notes, Loans and Financing Receivable [Line Items]
Extended chargeback period			84 days
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