Line of Credit - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Maximum amount of line of credit	$ 20,000,000
Line of credit facility expiration date	Dec. 31, 2019
Outstanding borrowings or letters of credit	$ 0	$ 0
Line of Credit Facility Expired December 2019 [Member]
Line of Credit Facility [Line Items]
Payment on unused portion of the loan	0.25%
Total fee	$ 50,000