Balance Sheet Components (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Inventories

Inventories

(in thousands)	December 31,		March 31,
	2018		2019
Raw materials	$	1,054	$	2,147
Finished products	4,690		5,909
	$	5,744	$	8,056

Inventories

(in thousands)	December 31,
	2017		2018
Raw materials	$	506	$	1,054
Finished products	2,742		4,690
	$	3,248	$	5,744

Property, Plant and Equipment

Property and Equipment, Net

(in thousands)	December 31,
	2017			2018
Furniture and fixtures	$	76		$	517
Equipment	1,059			1,217
Software	405			76
Leasehold improvements	189			1,978
	1,729			3,788
Less: Accumulated depreciation and amortization	(1,303		)	(946		)
Add: Construction-in-progress	60			38
	$	486		$	2,880

Schedule of Accrued Liabilities

Accrued Liabilities

(in thousands)	December 31,
	2017		2018
Accrued payroll and related expenses	$	2,718	$	5,157
Accrued professional services	64		1,014
Accrued royalty expense	—		313
Accrued travel expenses	68		270
Accrued clinical expenses	183		244
Accrued other expenses	76		588
	$	3,109	$	7,586