Condensed Consolidated Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash Flows Used In Operating Activities
Net loss	$ (23,687,000)	$ (13,206,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	125,000	124,000
Amortization of premiums (discounts) on investments	(210,000)	36,000
Stock-based compensation	1,683,000	1,881,000
Non-cash lease expense	329,000
Changes in operating assets and liabilities:
Receivables from collaborations	(516,000)	(682,000)
Prepaid expenses and other current assets	241,000	(234,000)
Other assets	(63,000)	(174,000)
Accounts payable	365,000	1,092,000
Accrued liabilities	1,461,000	1,366,000
Lease liability	(352,000)
Deferred revenue	0	(5,189,000)
Deferred rent, non-current		34,000
Net cash used in operating activities	(20,624,000)	(14,952,000)
Cash Flows Provided by Investing Activities
Purchases of investments	(14,104,000)	(16,832,000)
Proceeds from maturity of investments	23,100,000	38,026,000
Purchases of property and equipment		(26,000)
Net cash provided by investing activities	8,996,000	21,168,000
Cash Flows Provided By Financing Activities
Proceeds from issuance of common stock through an at-the-market offering, net	1,146,000
Proceeds from stock option exercises	8,000	150,000
Net cash provided by financing activities	1,154,000	150,000
Net (decrease) increase in cash, cash equivalents, and restricted cash	(10,474,000)	6,366,000
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	41,024,000	55,281,000
Supplemental Disclosure of Non-Cash Activities:
Unpaid amounts related to stock issuance and deferred financing costs	$ 29,000	$ 58,000