Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities
Net loss	$ (23,766)	$ (10,373)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,194	1,146
Depreciation and amortization	1,249	386
Accretion/amortization on available-for-sale marketable securities	(126)
Deferred rent costs	(395)	(142)
Changes in operating assets and liabilities:
Income tax receivable	(224)	(50)
Other receivables	(171)
Prepaid expenses	(1,669)	(303)
Accounts payable and accrued expenses	2,616	1,204
Deferred revenue		(1,363)
Net cash used in operating activities	(21,292)	(9,495)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	42,400
Proceeds from sale of available-for-sale marketable securities	10,900
Purchases of available-for-sale marketable securities	(42,677)
Change in restricted cash	(769)
Purchases of property and equipment	(158)	(12)
Net cash provided by (used in) investing activities	9,696	(12)
Financing activities
Proceeds from the exercise of stock options	40	35
Net cash provided by financing activities	40	35
Net cash decrease for the period	(11,556)	(9,472)
Cash and cash equivalents at beginning of period	15,101	52,663
Cash and cash equivalents at end of period	3,545	$ 43,191
Noncash investing and financing activities
Tenant improvements paid by landlord	$ 1,094