Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on Recurring Basis by Level within Fair Value Hierarchy (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets
Marketable securities	$ 69,003	$ 164,619
Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	17,527	9,708
Marketable securities	69,003	164,619
Financial Assets Total	86,530	174,327
Financial Liabilities
Derivative liability	56	83
Fair Value, Measurements, Nonrecurring
Financial Liabilities
Financial Liabilities Total	0
Level 1 | Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	2	1,880
Marketable securities	3,743	4,897
Financial Assets Total	3,745	6,777
Financial Liabilities
Derivative liability	0	0
Level 2 | Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	17,525	7,828
Marketable securities	65,260	159,722
Financial Assets Total	82,785	167,550
Financial Liabilities
Derivative liability	0	0
Level 3 | Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	0	0
Marketable securities	0	0
Financial Assets Total	0	0
Financial Liabilities
Derivative liability	$ 56	$ 83