Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (67,698)	$ (57,010)	$ (10,380)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	19,341	6,138	2,813
Depreciation	1,567	1,294	1,111
Amortization of premiums on investments	(279)	85
Loss on disposal of equipment		4
Changes in operating assets and liabilities:
Receivable from collaboration partner	1,013	43,987	(45,000)
Prepaid expenses and other current assets	(2,884)	(482)	(112)
Other long-term assets	(296)	(59)	(24)
Accounts payable	671	367	(434)
Accrued liabilities	8,945	2,968	3,250
Prepayment from collaboration partner	8,541	4,432
Other long-term liabilities	(178)	(120)	(109)
Deferred revenue	(33,558)	(11,442)	28,029
Net cash used in operating activities	(64,815)	(9,838)	(20,856)
Cash flow from investing activities:
Purchases of investments	(132,475)	(44,044)	(16,060)
Sales of investments	8,000	4,000
Maturities of investments	28,000	4,000
Purchases of property and equipment	(3,373)	(1,517)	(1,083)
Proceeds from sale of equipment		10
Net cash used in investing activities	(99,848)	(37,551)	(17,143)
Cash flow from financing activities:
Proceeds from issuance of common stock during follow-on offerings, net of issuance costs	181,863	133,862	61,148
Proceeds from exercise of stock options and employee stock purchase plan	6,208	2,365	506
Payments of prior period offering costs		(38)	(153)
Net cash provided by financing activities	188,071	136,189	61,501
Net increase in cash and cash equivalents	23,408	88,800	23,502
Cash, cash equivalents and restricted cash at beginning of period	224,857	136,057	112,555
Cash, cash equivalents and restricted cash at end of period	248,265	224,857	136,057
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	53	184	272
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 468	$ 283	103
Unpaid financing-related costs included in period-end accounts payable and accrued liabilities			$ 38