Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (74,013)	$ (58,125)	$ (57,280)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,478	5,793	2,800
Modification of equity awards	616	537
Depreciation & amortization	370	495	1,465
Amortization/accretion of available-for-sale marketable securities	(1,820)	(582)	(218)
Realized gain on sale of available-for-sale marketable securities	5	(5)	(23)
Realized gain on sale of property and equipment		(41)
Deferred rent costs	(156)	148	(114)
Deferred revenue	42,009
Changes in operating assets and liabilities:
Income tax receivable	67	121	(468)
Other receivables	(803)	(36)	(7)
Prepaid expenses	(3,170)	(105)	199
Accounts payable and accrued expenses	5,116	(3,027)	6,265
Net cash used in operating activities	(22,301)	(54,827)	(47,381)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	175,300	82,156	80,380
Proceeds from sale of available-for-sale marketable securities	79,808	8,755	34,003
Purchase of available-for-sale marketable securities	(337,854)	(127,394)	(68,648)
Purchases of property and equipment	(73)	(58)	(717)
Proceeds from sale of property and equipment		41
Net cash (used in) provided by investing activities	(82,819)	(36,500)	45,018
Financing activities
Proceeds from sale of common stock in a follow-on public offering, net of issuance costs	92,063	86,224
Proceeds from the sale of common stock under license agreement	14,556
Proceeds from the exercise of stock options	4,194	1,699	123
Net cash provided by financing activities	110,813	87,923	123
Net increase (decrease) in cash, cash equivalents and restricted cash	5,693	(3,404)	(2,240)
Cash, cash equivalents and restricted cash at beginning of period	10,157	13,561	15,801
Cash, cash equivalents and restricted cash at end of period	$ 15,850	$ 10,157	13,561
Noncash financing activities
Tenant improvements paid by landlord			$ 1,094