Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (54,804)	$ (45,290)	$ (31,390)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,280	1,465	691
Stock-based compensation expense	4,848	4,168	1,653
Noncash interest expense	564	354	112
Loss on extinguishment of debt	112
Change in fair value of warrants			1
Deferred rent	(250)	(119)	5
Changes in operating assets and liabilities:
Accounts receivable	42	(168)	(201)
Prepaid expenses and other assets	68	190	(1,217)
Inventories, net	(120)	(568)	(115)
Accounts payable	(61)	177	(208)
Accrued expenses and other liabilities	580	260	2,405
Deferred revenue	299	2,066	80
Net cash used in operating activities	(46,442)	(37,465)	(28,184)
Investing activities
Purchases and manufacture of property and equipment	(5,487)	(7,974)	(2,084)
Decrease in restricted cash		80
Net cash used in investing activities	(5,487)	(7,894)	(2,084)
Financing activities
Proceeds from issuance of common stock in public offering, net of offering costs	(385)	33,677	58,089
Proceeds from issuance of common stock and stock options exercises, net	1,018	1,804	153
Proceeds from private investment in public equity	39,723
Proceeds from notes payable, net of issuance costs	43,803	10,000	19,714
Repayments of notes payable	(32,404)	(309)	(4,037)
Net cash provided by financing activities	51,755	45,172	73,919
Net (decrease) increase in cash and cash equivalents	(174)	(187)	43,651
Cash and cash equivalents at beginning of period	73,662	73,849	30,198
Cash and cash equivalents at end of period	73,488	73,662	73,849
Supplemental disclosures of cash flow information
Cash paid for interest	2,732	1,506	515
Supplemental disclosures of noncash investing and financing activities
Accrued property and equipment	$ 82	247	128
Leasehold improvements paid by landlord		1,268	121
Public offering costs unpaid at year end		$ 420
Accretion of Series A-1, A-2, B, C, D and E redeemable convertible preferred stock to redemption value			4,570
Conversion of redeemable and convertible preferred stock to common stock			117,383
Conversion of preferred warrants to common stock			$ 1,226