Condensed Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (36,773)	$ (30,704)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	572	328
Stock-based compensation	6,649	5,184
Non-cash interest expense	729	0
Net accretion of discounts and amortization of premiums of available-for-sale securities	(71)	407
Changes in assets and liabilities
Accrued interest of available-for-sale securities	(50)	226
Prepaid expenses and other assets	730	(1,227)
Accounts receivable	760	424
Accounts payable	509	1,525
Accrued liabilities and other liabilities	325	(674)
Net cash used in operating activities	(26,620)	(24,511)
Cash flows from investing activities
Purchases of property and equipment	(621)	(1,244)
Purchases of available-for-sale securities	(40,071)	0
Maturities of available-for-sale securities	41,100	37,653
Net cash provided by investing activities	408	36,409
Cash flows from financing activities
Proceeds from issuance of long-term debt	34,600	0
Proceeds from public offerings, net of offering costs	0	61,822
Payment of debt issuance costs	(136)	0
Proceeds from issuance of common stock upon exercise of stock options	1,201	2,122
Proceeds from employee stock purchase plan	430	326
Net cash provided by financing activities	36,095	64,270
Net increase in cash and cash equivalents	9,883	76,168
Cash and cash equivalents at beginning of period	23,735	33,104
Cash and cash equivalents at end of period	33,618	109,272
Supplemental disclosure of noncash investing and financing activities
Debt issuance costs in accounts payable and accrued expense	460	0
Purchases of property and equipment in accounts payable and accrued expense	$ 51	$ 267