Senior Term Loan and Royalty Purchase Agreement - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2015	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2015
Line of Credit Facility [Line Items]
Long-term debt		$ 20,700,000
Loss on debt extinguishment			$ 0	$ 0	$ 0	$ 5,075,000
Credit Agreement
Line of Credit Facility [Line Items]
Repayments of debt		19,800,000
Payment of accrued interest		700,000
Prepayment penalty		1,500,000
Credit Agreement | Senior Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity (up to)	$ 40,000,000.0						$ 40,000,000.0
Proceeds from borrowing							$ 20,000,000
Debt instrument, stated interest rate	1.00%						1.00%
Basis spread (percent)	8.75%
Remaining borrowing capacity	$ 20,000,000						$ 20,000,000
Royalty Agreement
Line of Credit Facility [Line Items]
Extinguishment of debt		$ 4,500,000
Tier One | Royalty Agreement
Line of Credit Facility [Line Items]
Quarterly royalty payments, percentage of revenue	1.50%
Royalty agreement payment tier threshold	$ 50,000,000						50,000,000
Tier Two | Royalty Agreement
Line of Credit Facility [Line Items]
Quarterly royalty payments, percentage of revenue	2.45%
Royalty agreement payment tier threshold	$ 50,000,000						$ 50,000,000