Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (10,619)	$ (6,920)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	253	144
Stock-based compensation expense	722	239
Noncash interest expense	24	11
Change in fair value of warrants		(73)
Deferred rent	(18)	(6)
Changes in operating assets and liabilities:
Accounts receivable	113
Prepaid expenses and other current assets	390	(52)
Inventories	(137)
Accounts payable	438	93
Accrued expenses and other liabilities	(509)	773
Deferred revenue	1,949
Net cash used in operating activities	(7,394)	(5,791)
Investing activities
Purchases of property and equipment	(1,460)	(263)
Net cash used in investing activities	(1,460)	(263)
Financing activities
Proceeds from issuance of common stock in initial public offering, net of offering costs	375
Repayments of notes payable	(75)	(446)
Net cash provided by (used in) financing activities	300	(446)
Net (decrease) in cash and cash equivalents	(8,554)	(6,500)
Cash and cash equivalents at beginning of period	73,849	30,198
Cash and cash equivalents at end of period	65,295	23,698
Supplemental disclosures of cash flow information
Cash paid for interest	321	51
Supplemental disclosures of noncash investing and financing activities
Accrued costs of property and equipment	1,365
Accretion of Series A-1, A-2, B, C, D and E redeemable convertible preferred stock to redemption value		$ 1,906