Shareholders' Equity - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ (7,424)	$ 150,920
Issuance of common shares, net of offering costs	282,833
Issuance of common shares under equity incentive plan	2,607	1,065
Non-cash share-based compensation expense	16,879	7,330
Net loss	(172,937)	(62,304)
Ending balance	121,958	97,011
Common Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 881,426	$ 554,384
Beginning balance (in shares)	52,385,283	44,197,549
Issuance of common shares, net of offering costs	$ 282,833
Issuance of common shares, net of offering costs (shares)	5,555,554
Issuance of common shares under equity incentive plan	$ 10,880	$ 1,961
Issuance of common shares under equity incentive plan (shares)	447,111	85,445
Ending balance	$ 1,175,139	$ 556,345
Ending balance (in shares)	58,387,948	44,282,994
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 83,523	$ 40,104
Issuance of common shares under equity incentive plan	(8,273)	(896)
Non-cash share-based compensation expense	16,879	7,330
Ending balance	92,129	46,538
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(972,373)	(443,568)
Net loss	(172,937)	(62,304)
Ending balance	$ (1,145,310)	$ (505,872)