Marketable Investments - Summary of Maturity Periods of the Marketable Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
FY 2018	$ 21,764
FY 2019	26,720
FY2020	5,774
Total	54,258	$ 54,333
Federal Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
FY 2018	1,794
Total	1,794	1,793
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
FY 2018	19,970
FY 2019	26,720
FY2020	5,774
Total	$ 52,464	$ 52,540