Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (21,993)	$ (10,768)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	2,591	1,546
Net amortization of premiums and accretion of discounts on marketable debt securities	469	471
Depreciation and amortization	554	48
Realized gains on sales of marketable securities	(480)
Other non-cash adjustments	41	(13)
Changes in operating assets and liabilities
Accounts receivable	803	113
Prepaid expenses	(68)	(169)
Other current assets	88	(449)
Other assets	10	(55)
Accounts payable	2,468	(182)
Accrued expenses and other current liabilities	(1,920)	1,210
Advance payments		(92)
Deferred rent	(44)	453
Net cash used in operating activities	(17,481)	(7,887)
Cash flows from investing activities
Purchases of marketable securities	(5,188)	(20,429)
Maturities of marketable securities	10,500	10,106
Sales of marketable securities	780
Purchases of property and equipment	(2,929)	(416)
Net cash provided by (used in) investing activities	3,163	(10,739)
Cash flows from financing activities
Proceeds from exercise of stock options	161	21
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	71,299
Issuance costs for public offering of common stock	(84)
Net cash provided by financing activities	71,523	21
Net increase (decrease) in cash and cash equivalents	57,205	(18,605)
Cash and cash equivalents
Beginning of period	24,840	54,116
End of period	82,045	35,511
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses		$ 422
Issuance costs for public offering of common stock in accounts payable and accrued expenses	$ 328