Marketable Investments - Summary of Maturity Periods of the Marketable Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
FY 2018	$ 33,287
FY 2019	21,046
Total	54,333	$ 61,147
Federal Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
FY 2018	1,793
Total	1,793	1,793
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
FY 2018	31,494
FY 2019	21,046
Total	$ 52,540	$ 59,354