Cash, cash equivalents, and investments - Summary of Amortized Cost and Fair Value of Short-term and Long-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Due in less than one year	$ 28,983	$ 9,101
Due after one year, through two years	6,411	12,023
Amortized Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Due in less than one year	28,983
Due after one year, through two years	6,163
Due after two years	248
Total short-term and long-term investments	35,394
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Due in less than one year	28,989
Due after one year, through two years	6,167
Due after two years	242
Total short-term and long-term investments	$ 35,398