CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (44,199)	$ (40,172)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	332	87
Lease incentive	(17)	(17)
Stock-based compensation expense	3,767	2,496
Changes in operating assets and liabilities:
Accounts receivable	(2,748)	(2,952)
Inventories	(204)	(1,214)
Prepaid expenses and other assets	(1,083)	(351)
Accounts payable	(3,494)	2,537
Accrued expenses	1,074	3,308
Deferred revenue	5,417	3,926
Net cash used in operating activities	(41,155)	(32,352)
Investing activities
Purchase of intangible assets		(2,500)
Purchases of property and equipment	(478)	(25)
Net cash used in investing activities	(478)	(2,525)
Financing activities
Proceeds from issuances of common stock from public offerings, net of issuance costs of $526		51,174
Proceeds from issuances of common stock from employee stock purchase plans	673
Repayment of term note	(1,333)	(1,333)
Proceeds from the exercise of stock options	415	86
Payments made for employee restricted stock tax withholdings	(51)
Payments of offering costs for at-the-market equity program	(87)	(526)
Net cash provided by financing activities	(383)	49,927
Net (decrease) increase in cash and cash equivalents	(42,016)	15,050
Cash and cash equivalents at beginning of period	153,225	95,697
Cash and cash equivalents at end of period	111,209	110,747
Supplemental disclosure of cash flow information
Cash paid for offering costs	87	512
Cash paid for interest	88	$ 159
Supplemental disclosure of non-cash activities
Offering costs in accrued expenses	25
Acquisition of property and equipment in accrued expenses	$ 304