Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash Flows Used in Operating Activities
Net loss	$ (68,132,000)	$ (35,130,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	366,000	375,000
Amortization of premiums on investments	(730,000)	(124,000)
Stock-based compensation	5,008,000	5,617,000
Non-cash lease expense	1,012,000
Changes in operating assets and liabilities:
Receivables from collaborations	285,000	(667,000)
Prepaid expenses and other current assets	435,000	1,179,000
Other assets	208,000	(617,000)
Accounts payable	(534,000)	(563,000)
Accrued liabilities	2,813,000	1,164,000
Lease liability	(1,081,000)
Deferred revenue	0	(22,254,000)
Deferred rent, non-current		66,000
Net cash used in operating activities	(60,350,000)	(50,954,000)
Cash Flows (Used in) Provided by Investing Activities
Purchases of investments	(127,980,000)	(47,292,000)
Proceeds from sale and maturity of investments	102,050,000	104,870,000
Purchases of property and equipment	(8,000)	(184,000)
Net cash (used in) provided by investing activities	(25,938,000)	57,394,000
Cash Flows Provided by Financing Activities
Proceeds from issuance of common stock upon public offering, net	53,760,000
Proceeds from issuance of common stock through an at-the-market offering, net	2,523,000	5,603,000
Proceeds from stock option exercises and employee stock plan purchases	608,000	617,000
Net cash provided by financing activities	56,891,000	6,220,000
Net (decrease) increase in cash, cash equivalents, and restricted cash	(29,397,000)	12,660,000
Cash, cash equivalents, and restricted cash at beginning of period	51,498,000	48,915,000
Cash, cash equivalents, and restricted cash at end of period	$ 22,101,000	$ 61,575,000