Financial instruments - Summary of Short-Term Investments and Long-Term Investments of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 35,017	$ 24,195
Due within one year, Fair value	34,956	24,191
Due after one year through two years, Amortized cost	38,687	2,003
Due after one year through two years, Fair value	38,458	2,003
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	35,017	24,195
Gross unrealized gains		1
Gross unrealized losses	(61)	(5)
Fair value	34,956	24,191
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,014	12,199
Gross unrealized losses	(61)	(5)
Fair value	32,953	12,194
Short-term Investments [Member] | U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,003	11,996
Gross unrealized gains		1
Fair value	2,003	11,997
Long-term Investments [Member] | U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	38,687	2,003
Gross unrealized losses	(229)
Fair value	$ 38,458	$ 2,003