Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	$ 113,255	$ 159,488
Convertible note payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis		23,397
Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	15,152	37,072
Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	80,967	104,920
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	13,640	14,600
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	3,496	2,896
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	15,152	37,072
Fair Value, Inputs, Level 1 | Convertible note payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis		0
Fair Value, Inputs, Level 1 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	15,152	37,072
Fair Value, Inputs, Level 1 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	98,103	122,416
Fair Value, Inputs, Level 2 | Convertible note payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis		0
Fair Value, Inputs, Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 2 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	80,967	104,920
Fair Value, Inputs, Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	13,640	14,600
Fair Value, Inputs, Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	3,496	2,896
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Convertible note payable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities measured at fair value on a recurring basis		23,397
Fair Value, Inputs, Level 3 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	$ 0	$ 0