Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		$ 36,549		$ 66,248
Liabilities, fair value				1,156
U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]	6,800
Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]			296
Corporate Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]			3,106
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[2]	10,156		28,927
Restricted cash (money market funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		19,593	[3]	33,919	[4]
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		36,549		62,846
Fair Value, Inputs, Level 1 | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]	6,800
Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[2]	10,156		28,927
Fair Value, Inputs, Level 1 | Restricted cash (money market funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		$ 19,593	[3]	33,919	[4]
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value				3,402
Fair Value, Inputs, Level 2 | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]			296
Fair Value, Inputs, Level 2 | Corporate Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]			3,106
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value				1,156
Derivative liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	[5]			1,156
Derivative liability | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	[5]			$ 1,156

[1] Included in Short Term Investments on accompanying consolidated balance sheets.

[2] Included in Cash and Cash Equivalents on accompanying consolidated balance sheets.

[3] $19.4 million of restricted cash serves as full collateral for the redeemable convertible notes and $125,000 of restricted cash serves as collateral for the Company’s corporate credit card and deposit for its facility lease.

[4] $33.8 million of restricted cash serves as full collateral for the redeemable convertible notes and $125,000 of restricted cash serves as collateral for the Company’s corporate credit card and deposit for its facility lease.

[5] The fair value of the derivative liability is measured using the Black-Scholes option-pricing valuation model. Inputs used to determine the estimated fair value of the conversion option include the fair value of the underlying common stock at the valuation measurement date, the remaining contractual term of the conversion option, risk-free interest rates, expected dividends and expected volatility of the price of the underlying common stock. In addition, the Company estimated the convertible redeemable note exchange rate based on an analysis of its actual exchange of notes for cash redemption or exchange of notes for conversion to common stock. See Note 9 - Redeemable Convertible Notes for further detail.