Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (43,562)	$ (47,089)	$ (45,581)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	3,354	2,977	2,377
Stock-based compensation expense	11,369	9,038	6,278
Amortization (accretion) of discount or premium on short-term investments	198	(20)	270
Interest accrued on long-term debt	171	158	18
Conversion of accrued interest to long-term debt	1,472	1,357	1,067
(Gain) loss on disposal of property and equipment	15	(2)	3
Provision for bad debt	361	0	34
Provision for inventory obsolescence	866	822	248
Changes in operating assets and liabilities
Accounts receivable	2,277	(2,476)	(7,328)
Inventory	(8,742)	(5,857)	(5,602)
Prepaid expenses and other	(911)	72	1,199
Other assets	(367)	37	(7)
Accounts payable	(110)	869	(166)
Accrued liabilities	1,312	(40)	(80)
Accrued compensation and other employee benefits	295	281	1,242
Customer deposits	8,335	610	0
Deferred revenue	(29,161)	29,948	(127)
Deferred rent and other liabilities	1,171	3,236	2,793
Net cash used in operating activities	(51,657)	(6,079)	(43,362)
Investing activities
Purchases of property and equipment	(4,284)	(3,991)	(3,796)
Proceeds from sale of property and equipment	0	4	6
Proceeds from sale of short-term investments	3,600	4,700	3,000
Proceeds from maturity of short-term investments	79,599	34,800	57,309
Purchases of short-term investments	(81,405)	(65,774)	(32,750)
Net cash (used in) provided by investing activities	(2,490)	(30,261)	23,769
Financing activities
Proceeds from long-term debt	0	5,000	10,000
Repayment of long-term debt and lease financing obligations	(58)	(226)	(271)
Proceeds from sale of common stock, net	56,486	26,223	12,518
Issuance of common stock warrants	674	0	0
Proceeds from exercise of common stock warrants	0	0	2
Proceeds from issuance of common stock for employee stock purchase plan	1,793	1,489	1,295
Deferred offering costs	0	0	(152)
Tax withholdings related to net share settlements of restricted stock units	313	0	0
Proceeds from exercise of stock options	1,086	2,607	876
Net cash provided by financing activities	59,668	35,093	24,268
Net increase (decrease) in cash and cash equivalents	5,521	(1,247)	4,675
Effect of exchange rate changes on cash and cash equivalents	32	(26)	(42)
Cash and cash equivalents
Beginning of year	20,583	21,856	17,223
End of year	26,136	20,583	21,856
Supplemental disclosures
Cash paid for interest	4,416	4,071	2,844
Cash paid for taxes	154	217	69
Purchases of property and equipment, accrued but not paid	0	275	640
Rental instruments reclassified from inventory	1,023	801	772
Non-cash inventory exchange for services	0	28	112
Non-cash capital lease	0	0	48
Accrual of offering costs	$ 0	$ 0	$ 29