Fair Value - Quantitative Information about Inputs and Valuation Methodologies Used for Fair Value Measurements Classification (Detail) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Apr. 28, 2017 USD ($) $ / shares
Senhance Surgical Robotic System Acquisition | Significant Unobservable Inputs (Level 3) | Contingent Consideration | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.10
Senhance Surgical Robotic System Acquisition | Significant Unobservable Inputs (Level 3) | Contingent Consideration | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	0.12
Series B Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value | $	$ 2.2	$ 4.6	$ 6.2
Share price | $ / shares	$ 1.36	$ 2.26	$ 0.65
Probability of additional financing	100.00%	100.00%
Series B Warrant | Measurement Input, Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Term	2 years 9 months 18 days	3 years 3 months 18 days	5 years
Series B Warrant | Measurement Input, Risk Free Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk free rate	0.0171	0.0247	0.0181
Series B Warrant | Measurement Input, Price Volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk free rate	0.8234	0.876	0.7314