Description of Business and Basis of Financial Statements Description of Business and Basis of Financial Statements - Additional Details (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Accumulated deficit	$ 295,953	$ 243,825
Cash and cash equivalents	56,901	$ 59,991	$ 83,416	$ 102,238
Amount available for issuance	112,500
At-The-Market Sales Agreement
Class of Stock [Line Items]
Maximum aggregate offering price	$ 62,200