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Fair Value - Additional Information (Detail)
3 Months Ended 9 Months Ended
Apr. 28, 2017
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
$ / shares
Fair Value Measurements Disclosure [Line Items]            
Transfers of assets between Level 1, Level 2, and Level 3 of the fair value hierarchy   $ 0   $ 0   $ 0
Transfers of liabilities between Level 1, Level 2, and Level 3 of the fair value hierarchy   0   0   0
Warrants to purchase common shares | shares 24,900,000          
Offering price | $ / shares $ 1.00          
Change in fair value of warrant liabilities   8,760,000 $ 22,887,000 24,438,000 $ 25,213,000  
Series A Warrant [Member]            
Fair Value Measurements Disclosure [Line Items]            
Warrants, exercise price | $ / shares $ 1.00          
Warrant expiration date Oct. 31, 2017          
Series A Warrant [Member] | Black-Scholes Merton Model [Member]            
Fair Value Measurements Disclosure [Line Items]            
Estimated fair value of warrants $ 2,500,000          
Fair value input, share price | $ / shares $ 0.65          
Series A Warrant [Member] | Black-Scholes Merton Model [Member] | Measurement Input, Expected Term [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, term 1 year          
Series A Warrant [Member] | Black-Scholes Merton Model [Member] | Measurement Input, Risk Free Interest Rate [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, risk free rate 0.0107          
Series A Warrant [Member] | Black-Scholes Merton Model [Member] | Measurement Input, Expected Dividend Payment [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, risk free rate 0.00          
Series A Warrant [Member] | Black-Scholes Merton Model [Member] | Measurement Input, Price Volatility [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, risk free rate 0.7314          
Series B Warrant [Member]            
Fair Value Measurements Disclosure [Line Items]            
Warrants, exercise price | $ / shares $ 1.00          
Estimated fair value of warrants $ 6,200,000 $ 15,000,000   $ 15,000,000   $ 14,100,000
Fair value input, share price | $ / shares $ 0.65 $ 5.80   $ 5.80   $ 1.93
Series B Warrant [Member] | Measurement Input, Expected Term [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, term 5 years 3 years 6 months 29 days   3 years 6 months 29 days   4 years 3 months 29 days
Series B Warrant [Member] | Measurement Input, Risk Free Interest Rate [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, risk free rate 1.81 2.90   2.90   2.13
Series B Warrant [Member] | Measurement Input, Price Volatility [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value input, risk free rate 73.14 89.47   89.47   80.60
Common Stock [Member]            
Fair Value Measurements Disclosure [Line Items]            
Number of common stock or warrants in each unit | shares 1          
Common Stock [Member] | Series A Warrant [Member]            
Fair Value Measurements Disclosure [Line Items]            
Number of common stock or warrants in each unit | shares 1          
Common Stock [Member] | Series B Warrant [Member]            
Fair Value Measurements Disclosure [Line Items]            
Number of common stock or warrants in each unit | shares 0.75          
Senhance Surgical Robotic System Acquisition [Member]            
Fair Value Measurements Disclosure [Line Items]            
Fair value of contingent consideration       $ 100,000 $ 1,200,000