Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (31,504)	$ (23,766)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,426	1,194
Depreciation and amortization	245	1,249
Accretion/amortization on available-for-sale marketable securities	(163)	(126)
Realized gain on sale of available-for-sale marketable securities	(3)
Realized gain on sale of property and equipment	(13)
Deferred rent costs	(7)	(395)
Changes in operating assets and liabilities:
Income tax receivable	292	(224)
Other receivables	(88)	(171)
Prepaid expenses	(405)	(1,669)
Accounts payable and accrued expenses	(4,343)	2,616
Net cash used in operating activities	(33,563)	(21,292)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	35,906	42,400
Proceeds from sale of available-for-sale marketable securities	5,430	10,900
Purchases of available-for-sale marketable securities	(98,021)	(42,677)
Change in restricted cash		(769)
Purchases of property and equipment	(30)	(158)
Proceeds from sale of property and equipment	13
Net cash (used in) provided by investing activities	(56,702)	9,696
Financing activities
Proceeds from sale of common stock in a follow-on public offering, net of issuance costs	86,224
Proceeds from the exercise of stock options	1,365	40
Net cash provided by financing activities	87,589	40
Net cash decrease for the period	(2,676)	(11,556)
Cash and cash equivalents at beginning of period	12,092	15,101
Cash and cash equivalents at end of period	$ 9,416	3,545
Noncash investing and financing activities
Tenant improvements paid by landlord		$ 1,094