Summary of Significant Accounting Policies - Additional Information (Detail) € in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2019 EUR (€)	Jan. 01, 2019
Accounting Policies [Line Items]
Accumulated deficit	$ 552,095,000		$ 552,095,000		$ 509,406,000
Working capital	46,800,000		46,800,000
Short-term investments	9,973,000		9,973,000		51,790,000
Cash, EU insured amount | €							€ 0.1
Impairment charges			$ 0		0
Number of business segments | Segment			1
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	200,000
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	$ 203,378,000		$ 203,378,000		239,307,000
Weighted average discount rate	7.80%		7.80%				7.80%
Right-of-use asset	$ 2,600,000		$ 2,600,000
Operating lease, liability	2,927,000		2,927,000
Lease liability current	1,033,000		1,033,000		$ 0
Lease liability noncurrent	$ 1,900,000		$ 1,900,000
Weighted average remaining lease term for operating leases	3 years		3 years				3 years
Cash paid for operating leases			$ 700,000
Operating Expenses
Accounting Policies [Line Items]
Operating lease cost	$ 400,000		$ 700,000
U.S.
Accounting Policies [Line Items]
Percentage of total consolidated assets	42.00%		42.00%		54.00%		42.00%
Outside of U.S.
Accounting Policies [Line Items]
Percentage of total consolidated assets, excluding goodwill	43.00%		43.00%		34.00%		43.00%
Senhance Surgical Robotic System Acquisition | Europe
Accounting Policies [Line Items]
Assets	$ 118,400,000		$ 118,400,000		$ 111,000,000.0
Stock Options
Accounting Policies [Line Items]
Share based compensation, expense recognized			$ 6,300,000	$ 4,200,000
Patents
Accounting Policies [Line Items]
Amortization period			10 years
Minimum
Accounting Policies [Line Items]
Amortization period			5 years
Incremental borrowing rate								6.60%
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			93.00%		89.00%
Customer Concentration Risk | Sales | Two Customer
Accounting Policies [Line Items]
Concentration risk percentage			76.00%
Customer Concentration Risk | Sales | Three Customer
Accounting Policies [Line Items]
Concentration risk percentage		96.00%
Customer Concentration Risk | Sales | Five Customer
Accounting Policies [Line Items]
Concentration risk percentage	91.00%			94.00%
Geographic Concentration Risk | Sales | U.S.
Accounting Policies [Line Items]
Concentration risk percentage			5.00%	12.00%
Geographic Concentration Risk | Sales | Outside of U.S.
Accounting Policies [Line Items]
Concentration risk percentage			95.00%	88.00%
U.S. Government Securities
Accounting Policies [Line Items]
Short-term investments	$ 10,000,000.0		$ 10,000,000.0		$ 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