Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	[1]	$ 211,928	$ 563	$ 212,574	$ 3,009
Operating expenses:
Research and development	[2]	57,246	39,820	130,601	84,972
General and administrative		15,519	10,175	46,216	31,752
Total operating expenses		72,765	49,995	176,817	116,724
Income (loss) from operations		139,163	(49,432)	35,757	(113,715)
Other income (expense):
Loss from equity method investment		(3,430)	(1,012)	(5,467)	(3,256)
Other income (expense), net		2,964	(130)	6,470	(101)
Total other income (expense), net		(466)	(1,142)	1,003	(3,357)
Net income (loss) before income taxes		138,697	(50,574)	36,760	(117,072)
Provision for income taxes		(274)	(137)	(444)	(319)
Net income (loss)		138,423	(50,711)	36,316	(117,391)
Foreign currency translation adjustment		(12)	(6)	(14)	(15)
Comprehensive income (loss)		138,411	(50,717)	36,302	(117,406)
Reconciliation of net income (loss) to net income (loss) attributable to common shareholders:
Net income (loss)		$ 138,423	$ (50,711)	$ 36,316	$ (117,391)
Net income (loss) per share attributable to common shareholders— basic		$ 2.52	$ (1.07)	$ 0.68	$ (2.51)
Weighted-average common shares outstanding used in net income (loss) per share attributable to common shareholders—basic		54,829,057	47,391,988	53,380,123	46,709,388
Net income (loss) per share attributable to common shareholders— diluted		$ 2.40	$ (1.07)	$ 0.65	$ (2.51)
Weighted-average common shares outstanding used in net income (loss) per share attributable to common shareholders—diluted		57,598,901	47,391,988	55,821,420	46,709,388

[1] Including the following revenue from a related party, see Notes 6 & 11

[2] Including the following research and development expense with a related party, see Notes 6 & 11