Summary of Significant Accounting Policies - Chargebacks, Rebates, Administrative Fees and Other, and Sales Returns (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounting Policies [Abstract]
Payment terms period			90 days
Restricted cash	$ 803	$ 2,011	$ 803	$ 2,011	$ 1,800
Accounts, Notes, Loans and Financing Receivable [Line Items]
Extended chargeback period			84 days
Chargeback provision	$ 188,884	$ 236,701	$ 635,329	754,137
Sales chargebacks as percentage of gross sales	43.30%	40.40%
Gross sales	$ 436,700	$ 586,351	$ 1,465,052	1,833,998
Change in ratio of direct to indirect sales for chargeback reserves used in sensitivity analysis	4.50%		4.50%
Potential increase in chargeback expense based on sensitivity analysis	$ 300		$ 300
Potential decrease in chargeback expense based on sensitivity analysis	1,800		1,800
Rebates, administrative and other fees	$ 66,843	$ 127,904	$ 234,217	362,042
Rebates, administrative and other fees as a percentage of gross sales	15.30%	21.80%
Change in ratio of direct to indirect sales for rebates, administrative and other fees used in sensitivity analysis	4.50%		4.50%
Potential increase in rebates, administrative and other fees expense based on sensitivity analysis	$ 0		$ 0
Potential decrease in rebates, administrative and other fees expense based on sensitivity analysis	300		300
Sales returns	$ 4,805	$ 6,187	$ 18,059	$ 22,721
Change in ratio of direct to indirect sales for sales return reserve used in sensitivity analysis	1.10%	1.10%
Change in lag time to process returns for sales returns reserve used in sensitivity analysis			1 month
Potential increase in sales returns expense based on sensitivity analysis	$ 800		$ 800
Potential decrease in sales returns expense based on sensitivity analysis	$ 900		$ 900
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