Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Operating activities
Net loss	$ (77,400)	$ (43,562)	$ (47,089)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	4,070	3,354	2,977
Stock-based compensation expense	11,475	11,369	9,038
Paid in-kind interest on term loan agreements	(5,446)	0	0
Loss on extinguishment of long-term debt	842	0	0
Amortization (accretion) of discount or premium on short-term investments	278	198	(20)
Amortization of debt issuance costs and discounts	438	171	158
Conversion of accrued interest to long-term debt	1,530	1,472	1,357
(Gain) loss on disposal of property and equipment	97	15	(2)
Provision for bad debt	467	361	0
Provision for inventory obsolescence	691	866	822
Changes in operating assets and liabilities
Accounts receivable	1,807	2,277	(2,476)
Inventory	5,251	(8,742)	(5,857)
Prepaid expenses and other assets	(2,714)	(1,278)	109
Accounts payable	4,640	(110)	869
Accrued liabilities	(494)	1,312	(40)
Accrued compensation and other employee benefits	3,463	295	281
Customer deposits	(778)	8,335	610
Deferred revenue	(1,779)	(29,161)	29,948
Deferred rent and other liabilities	(503)	1,171	3,236
Net cash used in operating activities	(54,065)	(51,657)	(6,079)
Investing activities
Purchases of property and equipment	(4,485)	(4,284)	(3,991)
Proceeds from sale of property and equipment	0	0	4
Proceeds from sale of short-term investments	7,910	3,600	4,700
Proceeds from maturity of short-term investments	51,300	79,599	34,800
Purchases of short-term investments	(77,650)	(81,405)	(65,774)
Net cash used in investing activities	(22,925)	(2,490)	(30,261)
Financing activities
Proceeds from long-term debt	60,000	0	5,000
Deferred costs related to long-term debt	500	0	0
Repayment of long-term debt and lease financing obligations	(45,000)	(58)	(226)
Debt Instrument, Fee Amount	(1,009)	0	0
Proceeds from sale of common stock, net	53,829	56,486	26,223
Issuance of common stock for employee stock purchase plan	3,010	674	0
Proceeds from issuance of common stock for employee stock purchase plan	1,451	1,793	1,489
Tax withholdings related to net share settlements of restricted stock units	(207)	(313)	0
Proceeds from exercise of stock options	3,507	1,086	2,607
Net cash provided by financing activities	75,081	59,668	35,093
Net increase (decrease) in cash and cash equivalents	(1,909)	5,521	(1,247)
Effect of exchange rate changes on cash and cash equivalents	(14)	32	(26)
Cash and cash equivalents and restricted cash
Beginning of year	26,279	20,726	21,999
End of year	24,356	26,279	20,726
Cash and cash equivalents	24,356	26,136	20,583
Restricted Cash	0	143	143
Supplemental disclosures
Cash paid for interest	6,213	4,416	4,071
Fair value of warrants issued with long-term debt	1,583	0	0
Issuance of common stock warrants	4,593	674
Cash paid for taxes	231	154	217
Purchases of property and equipment, accrued but not paid	0	0	275
Rental instruments reclassified from inventory	585	1,023	801
Non-cash inventory exchanged for services	$ 106	$ 0	$ 28