Financial instruments - Summary of Short-Term Investments and Long-Term Investments of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 33,033	$ 35,017
Due within one year, Fair value	33,024	34,956
Due after one year through two years, Amortized cost	38,661	38,687
Due after one year through two years, Fair value	38,623	38,458
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		35,017
Gross unrealized losses		(61)
Fair value		34,956
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,033	33,014
Gross unrealized gains	2
Gross unrealized losses	(11)	(61)
Fair value	33,024	32,953
Short-term Investments [Member] | U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		2,003
Fair value		2,003
Long-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	38,661
Gross unrealized losses	(38)
Fair value	$ 38,623
Long-term Investments [Member] | U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		38,687
Gross unrealized losses		(229)
Fair value		$ 38,458