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Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Detail) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurements Using Significant Unobservable Inputs (Level 3) [Roll Forward]          
Beginning balance $ 560 $ 18,751 $ 18,751 $ 3,785 $ 225
Fair value of warrants issued 23,532 0 16,953 28,472 4,247
Fair value of warrants exercised 0 $ (19,000) (2,552) (726) (123)
Total change in the liability included in earnings (17,209)   (15,103) (12,780) (564)
Extinguishment of convertible note warrant     (17,489)    
Ending balance $ 6,883   $ 560 $ 18,751 $ 3,785