Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net loss	$ (35,231)	$ (17,322)	$ (1,673)
Adjustment to reconcile net loss to net cash used in operating activities:
Depreciation	4,534	3,950	2,869
Stock-based compensation	11,317	7,428	2,765
Non-cash restructuring benefit	(454)
Accretion of discount or amortization of premium on short-term investments	(419)	70	31
Revaluation of warrant liability			(559)
Amortization and write-off of debt issuance costs	75	95	239
Deferred income taxes	(395)
Changes in operating assets and liabilities:
Accounts receivable	3,782	(4,822)	(4,515)
Inventory	2,358	(19,349)	(15,897)
Prepaid expenses and other assets	473	439	(1,759)
Deferred revenue	(381)	(196)	17
Deferred rent	(260)	1,191	86
Accounts payable	326	(2,836)	3,883
Accrued compensation and employee related benefits	1,819	(1,735)	3,462
Accrued liabilities	(390)	(2,799)	1,554
Restructuring liabilities	1,069
Net cash used in operating activities	(11,777)	(35,886)	(9,497)
Investing activities:
Purchases of investments	(51,651)	(49,125)	(67,103)
Proceeds from maturities of investments	52,352	77,075
Purchases of property and equipment	(6,367)	(6,552)	(3,530)
Net cash provided by (used in) investing activities	(5,666)	21,398	(70,633)
Financing activities:
Proceeds from public offerings, net of offering costs			108,096
Payments on capital lease financing obligations	(900)	(1,147)	(1,229)
Payments on term loans	(2,451)	(2,772)	(65,320)
Proceeds from term loans, net of debt issuance costs	16,350		61,436
Proceeds from exercise of stock options and employee stock purchase plan	2,689	4,656	600
Proceeds from exercise of warrants			62
Payments of deferred offering costs		(600)
Net cash provided by financing activities	15,688	137	103,645
Net increase (decrease) in cash and cash equivalents	(1,755)	(14,351)	23,515
Cash and cash equivalents
Beginning of period	19,285	33,636	10,121
End of period	17,530	19,285	33,636
Supplemental disclosure of cash flow information:
Cash paid for interest	1,179	814	1,503
Supplemental disclosure of non-cash financing and investing activities:
Accretion on preferred stock			6,258
Vesting of early exercise stock options	139	183	167
Additions to property and equipment through capital lease			341
Accrued not yet paid offering costs			626
Purchases of property and equipment not yet paid	513	579	1,538
Write-off of fully depreciated property and equipment	$ 3,105	$ 592	1,571
Conversion of convertible preferred stock to common stock			106,084
Conversion of convertible preferred stock warrants to common stock warrants			$ 505