Financial instruments - Summary of Short-Term and Long-Term Investments of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 56,123	$ 70,819
Due within one year, Fair value	56,053	70,758
Due after one year through two years, Amortized cost		50,155
Due after one year through two years, Fair value		50,046
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	56,123	70,819
Gross unrealized gains	0	0
Gross unrealized losses	(70)	(61)
Fair value	$ 56,053	70,758
Long-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		50,155
Gross unrealized gains		0
Gross unrealized losses		(109)
Fair value		$ 50,046