Financial instruments - Summary of Short-Term Investments and Long-Term Investments of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 14,105	$ 24,195
Due within one year, Fair value	14,109	24,191
Due after one year through two years, Amortized cost		2,003
Due after one year through two years, Fair value		2,003
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	14,105	24,195
Gross unrealized gains	4	1
Gross unrealized losses		(5)
Fair value	14,109	24,191
Short-term investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	12,102	12,199
Gross unrealized gains	3
Gross unrealized losses		(5)
Fair value	12,105	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	1
Fair value	$ 2,004	11,997
Long-term investments [Member] | U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		2,003
Gross unrealized gains		0
Fair value		$ 2,003