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Summary of Significant Accounting Policies - Summary of Reconciliation of Denominator Used in Calculation of Basic and Diluted Loss Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share Basic And Diluted [Abstract]                      
Net loss $ (11,492) $ (18,663) $ (18,225) $ (15,448) $ (11,535) $ (10,583) $ (9,521) $ (8,075) $ (63,828) $ (39,714) $ (45,562)
Weighted-average common shares outstanding - basic                 38,529 36,827 35,197
Weighted-average common shares outstanding - diluted                 38,529 36,827 35,197
Net loss per common share, basic and diluted $ (0.29) $ (0.48) $ (0.48) $ (0.41) $ (0.31) $ (0.29) $ (0.26) $ (0.22) $ (1.66) $ (1.08) $ (1.29)