Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income (loss)	$ (17,322)	$ (1,673)	$ 900
Adjustment to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	3,950	2,869	1,972
Amortization and write-off of debt issuance costs	95	239	152
Amortization of premium on short-term investments	70	31
Revaluation of warrant liability		(559)	(703)
Stock-based compensation	7,428	2,765	1,178
Changes in operating assets and liabilities:
Accounts receivable	(4,822)	(4,515)	(3,327)
Inventory	(19,349)	(15,897)	(2,783)
Prepaid expenses and other assets	439	(1,759)	(360)
Deferred revenue	(196)	17	(466)
Deferred rent	1,191	86	4,601
Accounts payable	(2,836)	3,883	754
Accrued compensation and benefits	(1,735)	3,462	1,356
Accrued liabilities	(2,799)	1,554	182
Net cash provided by (used in) operating activities	(35,886)	(9,497)	3,456
Investing activities:
Purchases of investments	(49,125)	(67,103)
Proceeds from maturities of investments	77,075
Purchases of property and equipment	(6,552)	(3,530)	(7,450)
Net cash provided by (used in) investing activities	21,398	(70,633)	(7,450)
Financing activities:
Proceeds from initial public offering, net of offering costs		108,096
Payments on capital lease financing obligations	(1,147)	(1,229)	(824)
Payments on term loans	(2,772)	(65,320)	(6,115)
Proceeds from term loans		61,436	13,899
Proceeds from exercise of stock options and employee stock purchase plan	4,656	600	538
Proceeds from issuance of preferred stock upon exercise of warrants		62
Payments of deferred offering costs	(600)		(322)
Net cash provided by (used in) financing activities	137	103,645	7,176
Net increase (decrease) in cash and cash equivalents	(14,351)	23,515	3,182
Cash and cash equivalents:
Beginning of period	33,636	10,121	6,939
End of period	19,285	33,636	10,121
Supplemental disclosure of cash flow information:
Cash paid for interest	814	1,503	973
Supplemental disclosure of non-cash financing and investing activities:
Accretion on preferred stock		6,258	11,301
Vesting of early exercise stock options	183	167	117
Additions to property and equipment through capital lease		341	2,907
Accrued not yet paid offering costs		626	315
Purchases of property and equipment not yet paid	579	1,538	347
Write-off of fully depreciated property and equipment	$ 592	1,571	$ 1,913
Conversion of convertible preferred stock to common stock		106,084
Conversion of convertible preferred stock warrants to common stock warrants		$ 505