Financial instruments - Summary of Short-Term Investments and Long-Term Investments of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 100,817	$ 70,819
Due within one year, Fair value	100,665	70,758
Due after one year through two years, Amortized cost	20,061	50,155
Due after one year through two years, Fair value	20,008	50,046
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	100,817	70,819
Gross unrealized gains	0	0
Gross unrealized losses	(152)	(61)
Fair value	100,665	70,758
Long-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	20,061	50,155
Gross unrealized gains	0	0
Gross unrealized losses	(53)	(109)
Fair value	$ 20,008	$ 50,046