Available-for-Sale Debt Securities by Original Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
One year or less, amortized cost	$ 66,507	$ 85,049
Marketable equity securities, amortized cost	0	0
Greater than one year and less than five years, amortized cost	4,003
Amortized Cost	70,510	85,049
One year or less, fair value	66,512	85,000
Marketable equity securities, fair value	4,930	11,163
Greater than one year and less than five years, fair value	4,004
Total available-for-sale securities fair value	$ 75,446	$ 96,163