Changes in Deferred Revenue (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Revenue From Contract With Customer [Abstract]
Deferred revenue	$ 2,366	$ 2,833
Deferral of revenue billed in current period, net of recognition	1,992	1,719
Recognition of revenue deferred in prior periods	(1,749)	(1,877)
Deferred revenue	$ 2,609	$ 2,675