Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on Recurring Basis by Level within Fair Value Hierarchy (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financial Assets
Marketable securities	$ 189,876	$ 112,544
Financial Liabilities
Warrant liability	0	688
Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	27,054	21,552
Marketable securities	189,876	112,544
Financial Assets Total	216,930	134,096
Financial Liabilities
Derivative liability	1,660	5,914
Warrant liability		688
Financial Liabilities Total		6,602
Fair Value, Measurements, Nonrecurring
Financial Liabilities
Financial Liabilities Total	0
Level 1 | Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	20,701	15,683
Marketable securities	1,803	0
Financial Assets Total	22,504	15,683
Financial Liabilities
Derivative liability	0	0
Warrant liability		0
Financial Liabilities Total		0
Level 2 | Fair Value, Measurements, Recurring
Financial Assets
Cash equivalents	6,353	5,869
Marketable securities	188,073	112,544
Financial Assets Total	194,426	118,413
Financial Liabilities
Derivative liability	0	0
Warrant liability		0
Financial Liabilities Total		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	1,660	5,914
Warrant liability		688
Financial Liabilities Total		$ 6,602