Summary of Gross Unrealized Losses and Fair Values on Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Less than 12 months, Fair value	$ 44,032	$ 49,921
Less than 12 months, Gross unrealized loss	(786)	(411)
12 months or greater, Fair value	32,952	22,960
12 months or greater, Gross unrealized loss	$ (784)	$ (284)