Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (51,862)	$ (53,530)	$ (39,445)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	351	304	243
Amortization of premium/discount on investments	(26)	126	0
Stock-based compensation	8,633	9,283	6,297
Changes in operating assets and liabilities:
Accounts receivable	(130)	455	998
Inventory	(77)	(594)	12
Prepaid expenses	975	(892)	(1,021)
Restricted cash	(6,000)	0	0
Other receivables	22	(3,059)	0
Security deposit	(200)	0	0
Accounts payable	(2,225)	1,485	(3,829)
Accrued liabilities	1,203	976	(519)
Accrued litigation-related settlement	(1,250)	7,250	0
Deferred revenue	(2,545)	2,496	(510)
Deferred rent and other noncurrent liabilities	72	(40)	(27)
Net cash used in operating activities	(53,059)	(35,740)	(37,801)
Investing activities
Purchases of property and equipment	(428)	(133)	(427)
Purchases of short-term investments	(71,611)	(102,288)	0
Maturities of short-term investments	65,000	40,000	0
Net cash used in investing activities	(7,039)	(62,421)	(427)
Financing activities
Issuance of common stock in public offering, net of offering costs	86,624	30,310	75,359
Proceeds from issuances of common stock under stock-based compensation plans	714	144	436
Net cash provided by financing activities	87,338	30,454	75,795
Net (decrease) increase in cash and cash equivalents	27,240	(67,707)	37,567
Cash and cash equivalents, beginning of year	45,214	112,921	75,354
Cash and cash equivalents, end of year	72,454	45,214	112,921
Supplemental cash flow information
Unpaid stock offering costs	$ 48	$ 0	$ 0