Available-for-Sale Debt Securities by Original Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
One year or less, amortized cost	$ 31,278	$ 38,836
Greater than one year and less than five years, amortized cost	62,623	12,032
Amortized Cost	93,901	50,868
One year or less, fair value	31,175	38,781
Greater than one year and less than five years, fair value	62,300	11,990
Total available-for-sale securities fair value	$ 93,475	$ 50,771