Shareholders' Equity - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 121,958	$ (7,424)	$ 97,011	$ 150,920	$ (7,424)	$ 150,920
Issuance of common shares, net of offering costs		282,833	281,100
Issuance of common shares under equity incentive plan	11,748	2,607		1,065
Exercise of related party warrants			1,998
Exercise of stock options			148
Exercise of stock options (shares)					1,518,142
Non-cash share-based compensation expense	11,762	16,879	17,554	7,330
Net loss	(180,934)	(172,937)	(211,070)	(62,304)	$ (353,871)	(273,374)
Ending balance	(35,466)	121,958	186,741	97,011	(35,466)	186,741
Common Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 1,175,139	$ 881,426	$ 556,345	$ 554,384	$ 881,426	$ 554,384
Beginning balance (in shares)	58,387,948	52,385,283	44,282,994	44,197,549	52,385,283	44,197,549
Issuance of common shares, net of offering costs		$ 282,833	$ 281,100
Issuance of common shares, net of offering costs (shares)		5,555,554	6,976,745
Issuance of common shares under equity incentive plan	$ 23,866	$ 10,880		$ 1,961
Issuance of common shares under equity incentive plan (shares)	1,137,617	447,111		85,445
Exercise of related party warrants			$ 7,201
Exercise of related party warrants (shares)			215,000
Exercise of stock options			$ 320
Exercise of stock options (shares)			26,875
Ending balance	$ 1,199,005	$ 1,175,139	$ 844,966	$ 556,345	$ 1,199,005	$ 844,966
Ending balance (in shares)	59,525,565	58,387,948	51,501,614	44,282,994	59,525,565	51,501,614
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 92,129	$ 83,523	$ 46,538	$ 40,104	$ 83,523	$ 40,104
Issuance of common shares under equity incentive plan	(12,118)	(8,273)		(896)
Exercise of related party warrants			(5,203)
Exercise of stock options			(172)
Non-cash share-based compensation expense	11,762	16,879	17,554	7,330
Ending balance	91,773	92,129	58,717	46,538	91,773	58,717
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,145,310)	(972,373)	(505,872)	(443,568)	(972,373)	(443,568)
Net loss	(180,934)	(172,937)	(211,070)	(62,304)
Ending balance	$ (1,326,244)	$ (1,145,310)	$ (716,942)	$ (505,872)	$ (1,326,244)	$ (716,942)