CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (161,381)	$ (161,867)	$ (103,227)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	2,405	2,068	286
Share-based compensation expense	20,875	15,080	5,701
Class A common shares issued or to be issued as consideration for Primatope, including milestone payments		8,800
Loss on disposal of property and equipment		21	66
Other			235
Non-cash lease expense	1,400	1,211
Accretion of discounts on short-term investments	123	(3,501)	(1,423)
Deferred income taxes	4,361	(3,156)	(978)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,334)	(1,020)	(4,791)
Other long-term assets	(5,588)
Accounts payable	(4,969)	(4,705)	8,823
Accrued expenses and other liabilities	8,797	4,638	9,296
Accrued milestones		(15,000)	5,000
Operating lease liabilities	(1,699)	(1,264)
Other long-term liabilities	478	326
Net cash used in operating activities	(136,532)	(158,369)	(81,012)
Cash flows from investing activities:
Purchases of property and equipment	(283)	(3,203)	(5,290)
Purchases of short-term investments	(430,211)	(488,773)	(402,008)
Proceeds from the maturities of short-term investments	407,050	541,190	168,100
Net cash provided by investing activities	(23,444)	49,214	(239,198)
Cash flows from financing activities:
Payments of offering costs	(1,136)	(118)	(3,657)
Proceeds from issuance of common stock under incentive award plans and employee share purchase plan	6,553	1,119	377
Net cash provided by financing activities	227,086	84,107	346,736
Net increase in cash, cash equivalents and restricted cash	67,110	(25,048)	26,526
Cash, cash equivalents and restricted cash at beginning of period	47,138	72,186	45,660
Cash, cash equivalents and restricted cash at end of period	114,248	47,138	72,186
Supplemental information:
Cash paid for income taxes	482	1,724	383
Supplemental disclosure of non-cash investing and financing activities:
Right-of-use asset obtained in exchange for operating lease obligation	6,039
Deferred offering costs included in accrued expenses and accounts payable			404
Property and equipment included in accrued expenses and accounts payable		222	1,292
Series C convertible preferred shares
Cash flows from financing activities:
Proceeds from issuance of convertible preferred shares, net of issuance costs			190,822
Initial Public Offering | Class A common shares
Cash flows from financing activities:
Proceeds from issuance of common shares, net of underwriting discounts and commissions inclusive of the over-allotment option exercise			$ 159,194
Follow-On Offering | Class A common shares
Cash flows from financing activities:
Proceeds from issuance of common shares, net of underwriting discounts and commissions inclusive of the over-allotment option exercise	165,725	48,595
Private Placement | Class A1 common shares
Cash flows from financing activities:
Proceeds from issuance of common shares from private placement, net of placement agent fees	$ 55,944	$ 34,511