Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash Flows Provided By (Used In) Operating Activities
Net loss	$ (35,130)	$ (16,852)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	375	254
Amortization of premiums (discounts) on investments	(124)	356
Stock-based compensation	5,617	3,968
Loss on disposal of property and equipment		6
Changes in operating assets and liabilities:
Receivables from collaborations	(667)	(2,177)
Prepaid expenses and other current assets	1,179	(2,437)
Other assets	(617)	(468)
Accounts payable	(563)	767
Accrued liabilities	1,164	3,089
Deferred revenue	(22,254)	38,299
Deferred rent, non-current	66	309
Net cash provided by (used in) operating activities	(50,954)	25,114
Cash Flows Provided by (Used In) Investing Activities
Purchases of investments	(47,292)	(174,357)
Proceeds from maturity of investments	104,870	57,949
Purchases of property and equipment	(184)	(682)
Net cash provided by (used in) investing activities	57,394	(117,090)
Cash Flows Provided By Financing Activities
Proceeds from issuance of common stock upon public offering, net		75,386
Proceeds from issuance of common stock under stock purchase agreement, net		7,914
Proceeds from issuance of common stock through an at-the-market offering, net	5,603	36,907
Proceeds from stock option exercises and employee stock plan purchases	617	973
Net cash provided by financing activities	6,220	121,180
Net increase in cash, cash equivalents, and restricted cash	12,660	29,204	[1]
Cash, cash equivalents, and restricted cash at beginning of period	48,915	10,647	[1]
Cash, cash equivalents, and restricted cash at end of period	61,575	39,851	[1]
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Change in unpaid amounts related to property and equipment purchases	3	51
Change in unpaid amounts related to deferred financing costs	$ 8	$ 46

[1] For the nine months ended September 30, 2017, this line item, as originally reported, excluded restricted cash.