Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (77,187)	$ (38,924)	$ (36,957)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Stock-based compensation	8,353	6,919	4,241
Operating lease right-of-use asset amortization	1,792
Depreciation and amortization	703	527	406
Amortization of issuance costs and accretion of final payment fee for long-term debt	29
Gain (loss) on disposal of property and equipment	8		(62)
Net (accretion of discount) amortization of premium on available-for-sale securities	(594)	206	687
Change in deferred tax asset	(775)	(658)
Changes in operating assets and liabilities:
Research and development tax incentive receivable, net	1,411	(236)	1,070
Receivable from collaboration partner - related party	(2,168)	(2,771)	(1,816)
Prepaid expenses and other assets	(2,820)	1,117	(333)
Accounts payable	(3,000)	4,430	91
Payable to collaboration partner - related party	201	1,061
Accrued expenses and other payables	1,098	1,911	4,793
Deferred revenue - related party	33,307	(23,529)	31,752
Operating lease liability	(1,885)
Net cash (used in) provided by operating activities	(41,527)	(49,947)	3,872
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of marketable securities	(166,936)	(71,060)	(39,546)
Proceeds from maturities of marketable securities	114,193	73,759	56,035
Purchases of property and equipment, net	(967)	(486)	(666)
Net cash (used in) provided by investing activities	(53,710)	2,213	15,823
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from at-the-market offering, net of issuance costs	34,492	1,508
Proceeds from issuance of long-term debt, net of issuance costs	9,765
Proceeds from issuance of common stock upon exercise of stock options and purchases under employee stock purchase plan	1,779	934	1,007
Proceeds from issuance of common stock and warrants in private placement, net of issuance costs		21,673	64,547
Net cash provided by financing activities	46,036	24,115	65,554
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(26)	(177)	146
Net (decrease) increase in cash, cash equivalents and restricted cash	(49,227)	(23,796)	85,395
Cash, cash equivalents and restricted cash, beginning of period	82,693	106,489	21,094
Cash, cash equivalents and restricted cash, end of period	33,466	82,693	106,489
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for interest	70
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Purchases of property and equipment in accounts payable and accrued liabilities	100	24
Deferred offering costs in accounts payable and accrued liabilities	$ 80		66
Fair value of common stock retired in exchange for issuance of common stock warrant		$ 6,670
Acquisition of new equipment upon trade-in for existing equipment			$ 185