Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (33,258)	$ (22,315)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	979	459
Stock-based compensation expense	2,899	1,057
Noncash interest expense	288	102
Change in fair value of warrants		1
Deferred rent	(86)	15
Changes in operating assets and liabilities:
Accounts receivable	(177)
Prepaid expenses and other current assets	(28)	(901)
Inventories	(942)
Accounts payable	258	(227)
Accrued expenses and other liabilities	303	2,110
Deferred revenue	1,119
Net cash used in operating activities	(28,645)	(19,699)
Investing activities
Purchases of property and equipment	(6,100)	(1,039)
Decrease in restricted cash	80
Net cash used in investing activities	(6,020)	(1,039)
Financing activities
Proceeds from issuance of common stock in initial public offering, net of offering costs		60,145
Proceeds from issuance of common stock and stock option exercises	1,168	150
Proceeds from notes payable, net of issuance costs		9,800
Repayment of notes payable	(235)	(3,966)
Net cash provided by financing activities	933	66,129
Net (decrease) increase in cash and cash equivalents	(33,732)	45,391
Cash and cash equivalents at beginning of period	73,849	30,198
Cash and cash equivalents at end of period	40,117	75,589
Supplemental disclosures of cash flow information
Cash paid for interest	1,110	294
Supplemental disclosures of noncash activities
Accrued cost of property and equipment	$ 1,567
Accretion of Series A-1, A-2, B, C, D and E redeemable convertible preferred stock to redemption value		4,570
Issuance costs incurred but unpaid at period end		80
Initial public offering costs incurred but unpaid at period end		2,104
Conversion of redeemable convertible preferred stock to common stock		117,383
Conversion of preferred stock warrants to common stock		$ 1,226