Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019
Accounting Policies [Line Items]
Short-term investments	$ 22,904,000		$ 51,790,000
Impairment charges	$ 0		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
Contract assets included in accounts receivable	$ 200,000	$ 100,000
Revenue recognized, included in deferred revenue			300,000	$ 100,000
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
Number of business segments | Segment	1
Assets	$ 216,976,000		$ 239,307,000
Effect of change in accounting principle		11,000
Weighted average discount rate	8.00%
Right-of-use asset	$ 1,500,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentAssetsMember
Lease liability	$ 1,837,000
Lease liability current	$ 733,000
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesMember
Lease liability noncurrent	$ 1,100,000
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherNoncurrentLiabilitiesMember
Weighted average remaining lease term for operating leases	2 years 6 months 7 days
Cash paid for operating leases	$ 400,000
Operating Expenses [Member]
Accounting Policies [Line Items]
Operating lease cost	300,000
Accumulated Deficit [Member]
Accounting Policies [Line Items]
Effect of change in accounting principle	7,000	11,000
Accumulated Deficit [Member] | ASU 2018-07 [Member]
Accounting Policies [Line Items]
Effect of change in accounting principle	$ 7,000
U.S. [Member]
Accounting Policies [Line Items]
Percentage of total consolidated assets	47.00%		54.00%
International [Member]
Accounting Policies [Line Items]
Percentage of total consolidated assets, excluding goodwill	40.00%		34.00%
Senhance Surgical Robotic System Acquisition [Member] | Europe [Member]
Accounting Policies [Line Items]
Assets	$ 115,500,000		$ 111,000,000
Stock Options [Member]
Accounting Policies [Line Items]
Share based compensation, expense recognized	$ 3,000,000	$ 1,800,000
Patents [Member]
Accounting Policies [Line Items]
Amortization period	10 years
Minimum [Member]
Accounting Policies [Line Items]
Amortization period	5 years
Incremental borrowing rate					7.00%
Minimum [Member] | Developed Technology [Member]
Accounting Policies [Line Items]
Amortization period	5 years
Maximum [Member]
Accounting Policies [Line Items]
Amortization period	10 years
Incremental borrowing rate					8.50%
Maximum [Member] | Developed Technology [Member]
Accounting Policies [Line Items]
Amortization period	7 years
Customer Concentration Risk [Member] | Accounts Receivable [Member] | Five Customer [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage	90.00%		89.00%
Customer Concentration Risk [Member] | Sales [Member] | Five Customer [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage	81.00%
Customer Concentration Risk [Member] | Sales [Member] | Two Customer [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage		91.00%
Geographic Concentration Risk [Member] | Sales [Member] | U.S. [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage	6.00%	0.00%
Geographic Concentration Risk [Member] | Sales [Member] | Europe [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage	35.00%	100.00%
Geographic Concentration Risk [Member] | Sales [Member] | Asia [Member]
Accounting Policies [Line Items]
Concentration Risk Percentage	59.00%	0.00%
U.S. Government Securities [Member]
Accounting Policies [Line Items]
Short-term investments	$ 22,900,000		$ 51,800,000
Short-term investments, other-than-temporary impairment	0		0
Held In Cash Collateral Accounts [Member]
Accounting Policies [Line Items]
Restricted cash	$ 600,000		$ 600,000