Inventories - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Inventory Net Items Net Of Reserve Alternative [Abstract]
Inventory reserve balance	$ 200,000		$ 200,000		$ 500,000
Charges for excess and obsolete inventory	$ 0	$ 0	$ 11,000	$ 200,000