Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (47,089)	$ (45,581)	$ (50,038)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	2,977	2,377	1,590
Stock-based compensation expense	9,038	6,278	4,926
Loss on extinguishment of debt	0	0	581
(Accretion) amortization of discount or premium on short-term investments	(20)	270	86
Interest accrued on long-term debt	158	18	(348)
Conversion of accrued interest to long-term debt	1,357	1,067	420
(Gain) loss on disposal of property and equipment	(2)	3	0
Bad debt expense	0	34	0
Gain on sale of investments	0	0	(5)
Changes in operating assets and liabilities
Accounts receivable	(2,476)	(7,328)	(4,109)
Inventory	(5,035)	(5,354)	(1,252)
Prepaid expenses and other	72	1,199	(2,132)
Other assets	37	(7)	(70)
Accounts payable	869	(166)	21
Accrued liabilities	851	1,162	3,379
Deferred revenue	29,948	(127)	9,497
Deferred rent and other liabilities	3,236	2,793	(607)
Net cash used in operating activities	(6,079)	(43,362)	(38,061)
Investing activities
Purchases of property and equipment	(3,991)	(3,796)	(1,900)
Proceeds from sale of property and equipment	4	6	0
Proceeds from sale of short-term investments	4,700	3,000	4,500
Proceeds from maturity of short-term investments	34,800	57,309	35,977
Purchases of short-term investments	(65,774)	(32,750)	(62,911)
Decrease in restricted cash	0	0	59
Net cash provided by (used in) investing activities	(30,261)	23,769	(24,275)
Financing activities
Proceeds from long-term debt	5,000	10,000	30,000
Deferred costs related to long-term debt	0	0	(770)
Repayment of long-term debt and lease financing obligations	(226)	(271)	(18,214)
Proceeds from sale of common stock, net	26,223	12,518	57,015
Proceeds from exercise of common stock warrants	0	2	230
Proceeds from issuance of common stock for employee stock purchase plan	1,489	1,295	988
Repurchase of shares related to common stock warrant exercise	0	0	(94)
Deferred offering costs	0	(152)	0
Proceeds from exercise of stock options	2,607	876	411
Net cash provided by financing activities	35,093	24,268	69,566
Net increase (decrease) in cash and cash equivalents	(1,247)	4,675	7,230
Effect of exchange rate changes on cash and cash equivalents	(26)	(42)	52
Cash and cash equivalents
Beginning of year	21,856	17,223	9,941
End of year	20,583	21,856	17,223
Supplemental disclosures
Cash paid for interest	4,071	2,844	3,479
Cash paid for taxes	217	69	0
Purchases of property and equipment, accrued but not paid	275	640	0
Rental instruments reclassified from inventory	801	772	2,541
Non-cash inventory exchange for services	28	112	0
Non-cash capital lease	0	48	262
Accrual of offering costs	$ 0	$ 29	$ 0