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Description of Business and Basis of Presentation - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]      
Accumulated deficit $ (526,046)   $ (498,713)
Net cash used in operating activities $ (15,511) $ (10,057)