Financial instruments - Summary of Short-Term Investments and Long-Term Investments of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost	$ 20,132	$ 24,195
Due within one year, Fair value	20,135	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	20,132	24,195
Gross unrealized gains	3	1
Gross unrealized losses		(5)
Fair value	20,135	24,191
Short-term investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	12,133	12,199
Gross unrealized gains	1
Gross unrealized losses		(5)
Fair value	12,134	12,194
Short-term investments [Member] | U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	7,999	11,996
Gross unrealized gains	2	1
Fair value	8,001	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