Debt - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
	Sep. 22, 2016	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2016	Jan. 31, 2016
Line Of Credit Facility [Line Items]
Repayments of line of credit				$ 19,500,000	$ 5,000,000
Interest expense			$ 691,000	506,000	$ 538,000
Secured Debt
Line Of Credit Facility [Line Items]
Line of credit, outstanding amount			5,000,000
Western Alliance Bank
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 10,000,000
Line of credit, maturity date		Jun. 30, 2018
Line of credit facility, maximum borrowing capacity						$ 15,000,000	$ 5,000,000
Line of credit, outstanding amount			0	0
Unused borrowing availability			15,000,000	15,000,000
Repayment of term loan	$ 5,100,000
Repayment of full principal amount	5,000,000
Repayment of accrued interest	100,000
Repayments of line of credit	5,800,000
Interest expense	$ 100,000		$ 0	$ 500,000
Western Alliance Bank | Prime Rate
Line Of Credit Facility [Line Items]
Line of credit, interest rate		0.75%
Line of credit facility, fixed rate						3.25%