Shareholders' Equity - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
	Jan. 26, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 97,011	$ 150,920	$ 107,816	$ 131,971	$ 150,920	$ 131,971
Issuance of common shares upon completion of equity offering, net of offering costs		281,100		43,842	52,013
Exercise of warrants, net settlement of shares		1,998
Reclassification of warrant liability to equity	$ 5,203				5,203
Exercise of stock options		148	1,065	833	1,003
Non-cash share-based compensation expense		17,554	7,330	5,608	3,088
Net loss		(211,070)	(62,304)	(39,269)	(85,462)	(273,374)	(124,731)
Ending balance		186,741	97,011	118,830	107,816	186,741	118,830
Common Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 556,345	$ 554,384	$ 367,730	$ 311,061	$ 554,384	$ 311,061
Beginning balance (in shares)		44,282,994	44,197,549	38,607,017	36,057,748	44,197,549	36,057,748
Issuance of common shares upon completion of equity offering, net of offering costs		$ 281,100		$ 43,842	$ 52,013
Issuance of common shares upon completion of equity offering, net of offering costs (in shares)		6,976,745		1,111,111	2,000,000
Exercise of warrants, net settlement of shares (in shares)		215,000		261,140	228,219
Exercise of warrants, net settlement of shares		$ 7,201
Exercise of stock options		$ 320	$ 1,961	$ 1,653	$ 4,656
Exercise of stock options (in shares)		26,875	85,445	115,023	321,050
Ending balance		$ 844,966	$ 556,345	$ 413,225	$ 367,730	$ 844,966	$ 413,225
Ending balance (in shares)		51,501,614	44,282,994	40,094,291	38,607,017	51,501,614	40,094,291
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 46,538	$ 40,104	$ 28,194	$ 23,556	$ 40,104	$ 23,556
Exercise of warrants, net settlement of shares		(5,203)
Reclassification of warrant liability to equity					5,203
Exercise of stock options		(172)	(896)	(820)	(3,653)
Non-cash share-based compensation expense		17,554	7,330	5,608	3,088
Ending balance		58,717	46,538	32,982	28,194	58,717	32,982
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(505,872)	(443,568)	(288,108)	(202,646)	(443,568)	(202,646)
Net loss		(211,070)	(62,304)	(39,269)	(85,462)
Ending balance		$ (716,942)	$ (505,872)	$ (327,377)	$ (288,108)	$ (716,942)	$ (327,377)