Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		$ 135,737	$ 37,638
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]	45,949	14,334
Restricted cash (money market funds)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		175	5,333	[2]
U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[3]	89,613	16,471
Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[3]		1,500
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value		135,737	36,138
Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[1]	45,949	14,334
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		175	5,333	[2]
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	[3]	$ 89,613	16,471
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value			1,500
Fair Value, Inputs, Level 2 | Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	[3]		$ 1,500

[1] Included in cash and cash equivalents on accompanying condensed consolidated balance sheets.

[2] $5.2 million of restricted cash in the Indenture serves as full collateral for the redeemable convertible notes and $0.1 million of restricted cash serves as collateral for the Company’s corporate credit card and deposit for its facility lease.

[3] Included in short-term investments on accompanying condensed consolidated balance sheets and are classified as available-for-sale securities.