Summary of Significant Accounting Policies - Additional Information (Detail) € in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 EUR (€)	Jan. 01, 2019
Accounting Policies [Line Items]
Accumulated deficit	$ 649,868,000			$ 649,868,000		$ 509,406,000
Working capital	34,400,000			34,400,000
Short-term investments	0			0		51,790,000
Cash, EU insured amount | €								€ 0.1
Provision for doubtful accounts	1,600,000			$ 1,630,000	$ 0
Number of business segments | Segment				1
Goodwill impairment	78,969,000		$ 0	$ 78,969,000	0	0
Impairment of intangible assets	7,912,000		0	$ 7,912,000	0
Period of service sale arrangement				5 years
Period of service sale arrangement at stated service price				4 years
Product warranty				1 year
Revenue, remaining performance obligation	3,800,000			$ 3,800,000
Contract assets included in accounts receivable	200,000		$ 100,000	200,000	100,000
Revenue recognized, included in deferred revenue		$ 1,300,000		700,000	300,000
Write down of inventory				$ 761,000	0
Statutory income tax rate				21.00%
Decrease in deferred tax assets due to newly enacted tax rate							$ 36,100,000
Decrease in valuation allowance due to newly enacted tax rate							$ 36,100,000
Assets	$ 97,634,000			$ 97,634,000		239,307,000
Weighted average discount rate	7.80%			7.80%				7.80%
Right-of-use asset	$ 2,400,000			$ 2,400,000
Operating lease, liability	2,632,000			2,632,000
Lease liability current	1,042,000			1,042,000		$ 0
Lease liability noncurrent	$ 1,600,000			$ 1,600,000
Weighted average remaining lease term for operating leases	2 years 9 months 18 days			2 years 9 months 18 days				2 years 9 months 18 days
Cash paid for operating leases				$ 1,300,000
Operating Expenses
Accounting Policies [Line Items]
Operating lease cost	$ 300,000			$ 1,000,000.0
U.S.
Accounting Policies [Line Items]
Percentage of total consolidated assets	28.00%			28.00%		54.00%		28.00%
Outside of U.S.
Accounting Policies [Line Items]
Percentage of total consolidated assets, excluding goodwill	72.00%			72.00%		34.00%		72.00%
Senhance Surgical Robotic System Acquisition | Europe
Accounting Policies [Line Items]
Assets	$ 70,500,000			$ 70,500,000		$ 111,000,000.0
Stock Options
Accounting Policies [Line Items]
Share based compensation, expense recognized				$ 9,700,000	$ 6,700,000
Patents
Accounting Policies [Line Items]
Amortization period				10 years
Impairment of intangible assets				$ 0
Minimum
Accounting Policies [Line Items]
Amortization period				5 years
Incremental borrowing rate									6.10%
Minimum | Developed Technology
Accounting Policies [Line Items]
Amortization period				5 years
Maximum
Accounting Policies [Line Items]
Amortization period				10 years
Incremental borrowing rate									8.50%
Maximum | Developed Technology
Accounting Policies [Line Items]
Amortization period				7 years
Customer Concentration Risk | Accounts Receivable | Five Customer
Accounting Policies [Line Items]
Concentration risk percentage				90.00%		89.00%
Customer Concentration Risk | Sales | Five Customer
Accounting Policies [Line Items]
Concentration risk percentage	83.00%			83.00%	68.00%
Customer Concentration Risk | Sales | Four Customer
Accounting Policies [Line Items]
Concentration risk percentage			96.00%
Geographic Concentration Risk | Sales | U.S.
Accounting Policies [Line Items]
Concentration risk percentage				5.00%	16.00%
Geographic Concentration Risk | Sales | Outside of U.S.
Accounting Policies [Line Items]
Concentration risk percentage				95.00%	84.00%
U.S. Government Securities
Accounting Policies [Line Items]
Short-term investments	$ 0.0			$ 0.0		$ 51,800,000
Short-term investments, other-than-temporary impairment				0		0
Held In Cash Collateral Accounts
Accounting Policies [Line Items]
Restricted cash	$ 700,000			$ 700,000		$ 600,000