Long-Term Loan, Net of Discount and Issuance Costs - Additional Information (Details) - Term Loan - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2016
Line Of Credit Facility [Line Items]
Term loan agreement, maturity period	5 years
Line of credit facility, maximum borrowing capacity	$ 100,000,000
Line of credit facility, current borrowing capacity	$ 25,000,000
Line of credit facility, remaining borrowing capacity				$ 75,000,000
Line of credit facility, drawdown received date			2016-07
Term loan credit facility, outstanding principal		$ 100,000,000	$ 100,000,000
Interest on the term loan credit facility		10.00%
Funding fees payable		1.50%
Prepayment fee percent		0.75%
Due date of outstanding principal loan		Jan. 31, 2020
Frequency of payments		Quarterly
Other Long-term Assets
Line Of Credit Facility [Line Items]
Total discount		$ 1,204,000	1,699,000
Additional deferred issuance costs		$ 1,454,000	$ 2,070,000
First Year
Line Of Credit Facility [Line Items]
Prepayment fee percent		3.00%
Second Year
Line Of Credit Facility [Line Items]
Prepayment fee percent		2.00%
Third Year
Line Of Credit Facility [Line Items]
Prepayment fee percent		1.00%