Revolving Credit Facility - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2013
Line of Credit Facility [Line Items]
Amount drawn under credit facility		$ 5,000,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Additional borrowing credit facility			$ 5,000,000
Total credit facility borrowing capacity			20,000,000
Revolving credit facility period	3 years
Variable interest rate	The LIBOR Rate plus 1.5%
LIBOR Rate plus	1.50%
Annual commitment fee	0.15%
Due date of amounts outstanding under credit facility	Dec. 21, 2018
Amount drawn under credit facility	$ 5,000,000
Borrowing credit facility			$ 15,000,000	$ 10,000,000