Goodwill and Acquired Intangible Assets - Changes in the Carrying Amount of Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
Balance as of December 31, 2016	$ 37,448	[1]
Goodwill disposed with sale of Skyfence	(1,059)
Balance as of September 30, 2017	$ 36,389

[1] The Condensed Consolidated Balance Sheet as of December 31, 2016 has been derived from the audited consolidated financial statements at that date but does not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements.