Available-for-Sale Debt Securities by Original Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
One year or less, amortized cost	$ 43,261	$ 54,131
Marketable equity securities, amortized cost	0	0
Greater than one year and less than five years, amortized cost	6,015
Amortized Cost	49,276	54,131
One year or less, fair value	43,244	54,107
Marketable equity securities, fair value		3,952
Greater than one year and less than five years, fair value	6,004
Total available-for-sale securities fair value	$ 49,248	$ 58,059