CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (57,462)	$ (66,617)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	464	360
Lease incentive	(26)	(25)
Stock-based compensation expense	5,867	4,137
Changes in operating assets and liabilities:
Accounts receivable	(3,694)	(1,679)
Inventories	(85)	(1,576)
Prepaid expenses and other assets	(33)	(114)
Accounts payable	(4,124)	5,401
Accrued expenses	10,315	4,409
Deferred revenue	(4,944)	3,938
Net cash used in operating activities	(53,722)	(51,766)
Investing activities
Purchase of intangible assets		(2,500)
Purchases of property and equipment	(818)	(136)
Net cash used in investing activities	(818)	(2,636)
Financing activities
Proceeds from issuances of common stock from public offerings, net of issuance costs of $507 and $526	9,425	51,619
Proceeds from issuances of common stock from employee stock purchase plans	1,141
Repayment of term note	(2,000)	(2,000)
Proceeds from the exercise of stock options	428	114
Payments made for employee restricted stock tax withholdings	(68)
Net cash provided by financing activities	8,926	49,733
Net (decrease) increase in cash and cash equivalents	(45,614)	(4,669)
Cash and cash equivalents at beginning of period	153,225	95,697
Cash and cash equivalents at end of period	107,611	91,028
Supplemental disclosure of cash flow information
Cash paid for offering costs	447	512
Cash paid for interest	121	$ 226
Supplemental disclosure of non-cash activities
Offering costs in accrued expenses	60
Receivable from issuance of common stock from at-the-market offering in other current assets	2,090
Acquisition of property and equipment in accrued expenses	$ 97