Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (39,501)	$ (43,277)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	4,044	3,992
Change in deferred tax asset	1,412	(1,446)
Operating lease right-of-use asset amortization	887	903
Loss on early repayment of debt	585
Depreciation and amortization	419	306
Amortization of debt issuance costs and accretion of debt discount	22
Accretion of discount on marketable securities, net of premium amortization	(243)	(209)
Changes in operating assets and liabilities:
Research and development tax incentive receivable	(278)
Receivable from collaboration partner - related party	3,758	(302)
Prepaid expenses and other assets	(248)	(1,804)
Accounts payable	73	(2,998)
Payable to collaboration partner - related party	(259)	(251)
Accrued expenses and other payables	382	(319)
Deferred revenue - related party	(7,517)	33,344
Operating lease liability	(958)	(931)
Other liability	92
Net cash used in operating activities	(37,330)	(12,992)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from maturities of marketable securities	104,583	44,400
Purchase of marketable securities	(66,753)	(52,459)
Purchases of property and equipment	(271)	(1,058)
Net cash provided by (used in) investing activities	37,559	(9,117)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from public offering of common stock, net of issuance costs	105,689
Proceeds from at-the-market offering, net of issuance costs	16,834	10,543
Proceeds from issuance of common stock upon under equity incentive and employee stock purchase plans	991	477
Issuance costs related to long-term debt	(14)
Early repayment of long-term debt	(10,524)
Net cash provided by financing activities	112,976	11,020
Effect of exchange rate changes on cash, cash equivalents and restricted cash	31	(1)
Net increase (decrease) in cash, cash equivalents and restricted cash	113,236	(11,090)
Cash, cash equivalents and restricted cash, beginning of period	33,466	82,693
Cash, cash equivalents and restricted cash, end of period	146,702	71,603
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Purchases of property and equipment in accounts payable and accrued liabilities	21	82
Tenant improvement allowance reimbursement		$ 469
Public Offering | Accrued liabilities and other payables
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Issuance costs	233
Public Offering | Prepaid expenses and other assets
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Issuance costs	125
At-the-market offering | Prepaid expenses and other assets
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Issuance costs	$ 191