Goodwill and Intangible Assets - Schedule of Change in Gross Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	$ 75,110	$ 74,565
Goodwill, ending balance	83,164	75,110
NT OBJECTives, Inc. [Member]
Goodwill And Intangible Assets [Line Items]
Acquisition		$ 545
Logentries [Member]
Goodwill And Intangible Assets [Line Items]
Acquisition	$ 8,054