Stock-Based Compensation - Schedule of Employee Stock Purchase Plan Activity (Detail) - ESPP [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares reserved	297,375	288,868	281,435
Shares issued	110,040	73,262	72,568
Shares available for future issuance	1,023,970	836,635	621,029
Employee contributions for shares issued	$ 5,521	$ 4,697	$ 3,499