Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (31,003)	$ (31,358)	$ (33,704)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,841	3,511	2,254
Bad debt expense	0	68	105
Loss on disposal of property and equipment	12	12	0
Genzyme co-promotion fee amortization	0	(948)	(1,897)
Stock-based compensation	6,617	6,378	5,602
Conversion of accrued interest to long-term debt	0	385	0
Amortization and write-off of debt discount and issuance costs	472	173	46
Interest on end-of-term debt obligation and prepayment penalty	1,589	206	79
Changes in operating assets and liabilities:
Accounts receivable	(3,960)	(5,321)	(558)
Supplies inventory	(1,849)	292	(71)
Prepaid expenses and current other assets	(7)	(415)	304
Other assets	(192)	25	(42)
Accounts payable	1,728	(1,441)	(3,546)
Accrued liabilities and deferred rent	(1,163)	451	4,463
Net cash used in operating activities	(23,915)	(27,982)	(26,965)
Investing activities
Purchases of property and equipment	(1,755)	(4,210)	(6,165)
Proceeds from the sale of property and equipment	440	0	0
Change in restricted cash	120	(2)	(533)
Net cash used in investing activities	(1,195)	(4,212)	(6,698)
Financing activities
Proceeds from the issuance of long-term debt, net of debt issuance costs	24,880	24,452	0
Proceeds from issuance of common stock in a private placement, net of issuance costs	0	0	37,258
Proceeds from issuance of common stock in a public offering, net of issuance costs	200	31,949	0
Payment of long-term debt	(25,385)	(5,000)	0
Payment of end-of-term debt obligation and prepayment penalty	(1,536)	(288)	0
Payment of deferred stock offering costs	0	0	(247)
Payment of capital lease liability	(274)	0	0
Proceeds from the exercise of common stock options and employee stock purchases	1,897	1,216	722
Net cash (used in) provided by financing activities	(218)	52,329	37,733
Net (decrease) increase in cash and cash equivalents	(25,328)	20,135	4,070
Cash and cash equivalents at beginning of year	59,219	39,084	35,014
Cash and cash equivalents at end of year	33,891	59,219	39,084
Supplementary cash flow information of non-cash investing and financing activities:
Net receivable for reimbursement of public offering issuance costs	0	144	0
Purchases of property and equipment included in accounts payable and accrued liabilities	42	363	1,825
Supplementary cash flow information:
Cash paid for interest on debt	2,718	2,149	278
Cash paid for tax	$ 21	$ 7	$ 22