Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Money market funds	$ 2,000
Cash and cash equivalents	49,458		$ 93,526	$ 140,527	$ 111,810
Adjustment to accumulated deficit	$ 1,041,336		$ 998,538
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to accumulated deficit		$ 600