Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (36,466)	$ (25,205)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	5,074	3,202
Net amortization of premiums and accretion of discounts on marketable debt securities	945	987
Depreciation and amortization	1,257	104
Realized gains on sales of marketable securities	(480)
Other non-cash adjustments	40
Changes in operating assets and liabilities
Accounts receivable	982	1,353
Prepaid expenses	(657)	(460)
Other current assets	(242)	(586)
Other assets	(86)	(98)
Accounts payable	2,723	930
Accrued expenses and other current liabilities	(31)	2,982
Advance payments		(127)
Deferred rent	(89)	601
Net cash used in operating activities	(27,030)	(16,317)
Cash flows from investing activities
Purchases of marketable securities	(46,593)	(32,261)
Maturities of marketable securities	28,010	26,131
Sales of marketable securities	780
Purchases of property and equipment	(4,609)	(1,491)
Restricted cash		(225)
Net cash used in investing activities	(22,412)	(7,846)
Cash flows from financing activities
Proceeds from exercise of stock options	329	130
Proceeds from issuance of common stock under employee stock purchase plan	147
Proceeds from public offering of common stock, net of underwriting discounts and commissions	81,994
Issuance costs for public offering of common stock	(219)
Net cash provided by financing activities	82,251	130
Net increase (decrease) in cash and cash equivalents	32,809	(24,033)
Cash and cash equivalents
Beginning of period	24,840	54,116
End of period	57,649	30,083
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses		$ 939
Issuance costs for public offering of common stock in accounts payable and accrued expenses	$ 193