Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Realized gain (loss) on sale of marketable securities	$ (1,894,000)	$ 6,977,000
Marketable securities classified as cash equivalents, maximum original maturity	90 days
Policy for marketable securities	90 days
Deferred revenue, current	$ 41,705,000	33,540,000
Deferred revenue, net of current portion	43,075,000	49,392,000
Significant interest and penalty expense related to uncertain tax positions	$ 0
Number of reporting segment | Segment	1
Scenario, Forecast | Reduction In Deferred Revenue
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit				$ (68,300,000)
Scenario, Forecast | Reduction In Deferred Tax Asset
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit				$ (13,600,000)
Research and Development
Significant Accounting Policies [Line Items]
License fee	$ 7,700,000	$ 2,400,000	$ 3,500,000