Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (72,176)	$ (50,636)	$ (44,995)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,197	1,708	1,256
Stock-based compensation	5,319	2,907	1,066
Accretion on asset retirement obligation	40	36	8
Change in fair value of warrant liability	16,598	(3)	(45)
Loss on disposal of property and equipment	9	358	12
Inventory lower of cost and market adjustment	911	1,939	2,578
Amortization of debt discount and interest accrual	3,321	2,629	1,129
Write-off of deferred offering cost	0	0	2,920
Changes in operating assets and liabilities:
Accounts receivable	(16,126)	(3,370)	74
Inventory	(12,329)	(2,065)	(2,413)
Deposits on purchased inventory	(4,521)	1,414	(1,138)
Deferred cost of revenue	(9,787)	4,873	(4,070)
Prepaid expenses and other assets	(2,044)	(1,633)	(733)
Accounts payable	6,309	381	(2,053)
Notes payable	0	0	(240)
Accrued expenses and other long-term liabilities	850	2,197	1,532
Customer deposits and deferred revenue	11,376	11,109	5,263
Net cash used in operating activities	(70,053)	(28,156)	(39,849)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(2,163)	(7,031)	(4,151)
Purchase of intangible and other assets	0	(12)	(104)
Change in restricted cash balance	0	(200)	110
Net cash used in investing activities	(2,163)	(7,243)	(4,145)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of convertible preferred stock, net	0	0	15,729
Proceeds from draw down of long-term debt	0	15,000	30,000
Payment of debt issuance cost	0	(18)	(1,132)
Payments of long-term debt	0	0	(15,000)
Proceeds from common stock private placement, gross	26,100	13,750	29,447
Payments of costs related to the initial public offering	0	0	(2,728)
Proceeds from the exercise of stock options	665	539	24
Proceeds from the exercise of warrants	103	0	0
Net cash provided by financing activities	115,407	28,930	53,532
NET INCREASE (DECREASE) IN CASH	43,191	(6,469)	9,538
CASH AND CASH EQUIVALENTS — BEGINNING OF PERIOD	14,198	20,667	11,129
CASH AND CASH EQUIVALENTS — END OF PERIOD	57,389	14,198	20,667
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Cash paid for interest	3,925	3,310	2,332
Cash paid for taxes	1	0	1
SUPPLEMENTAL NON-CASH INVESTING AND FINANCING ACTIVITIES:
Fair value of common stock warrants issued to placement agents as payment for service	0	0	316
Fair value of common stock warrants reclassed from liability to additional paid-in capital upon exercise	274	0	0
Transfer of property and equipment from inventory	125	117	0
Purchase of property and equipment in accounts payable and accrued expenses	96	193	1,136
Offering costs included in accounts payable and accrued expenses	0	189	0
Conversion of convertible preferred stock warrants into common stock warrants in connection with the Merger	0	0	160,839
Asset retirement obligation	0	0	258
Conversion of convertible preferred stock warrants into common stock warrants	0	0	93
Private Placement
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of offering costs related to common stock issuance	(300)	(341)	(2,808)
At The Market Offering Program
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of offering costs related to common stock issuance	(1,147)	0	0
Proceeds from offering of common stock, gross	40,126	0	0
Direct Registered Offering
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of offering costs related to common stock issuance	(81)	0	0
Proceeds from offering of common stock, gross	$ 49,941	$ 0	$ 0