Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock/Ordinary Shares	Additional paid in capital	Accumulated other comprehensive income	Accumulated deficit	Total
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 670	$ 461,911	$ 27	$ (393,978)	$ 68,630
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018	67,019
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 43	10,014			10,057
Issuance of ordinary shares (in shares)	4,317
Equity transaction costs		(270)			(270)
Stock-based compensation expense		1,907			1,907
Net loss				(20,217)	(20,217)
Stockholders' equity, ending balance at Mar. 31, 2019	$ 713	473,562	27	(414,195)	60,107
Stockholders' equity, ending balance (in shares) at Mar. 31, 2019	71,336
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 670	461,911	27	(393,978)	68,630
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018	67,019
Consolidated Statements of Changes in Stockholders' Equity
Net loss					(59,719)
Stockholders' equity, ending balance at Sep. 30, 2019	$ 780	492,105	27	(453,697)	39,215
Stockholders' equity, ending balance (in shares) at Sep. 30, 2019	77,994
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 670	461,911	27	(393,978)	68,630
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018	67,019
Stockholders' equity, ending balance at Dec. 31, 2019	$ 945	517,044	27	(476,742)	41,274
Stockholders' equity, ending balance (in shares) at Dec. 31, 2019	94,545
Stockholders' equity, beginning balance at Mar. 31, 2019	$ 713	473,562	27	(414,195)	60,107
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2019	71,336
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 16	3,691			3,707
Issuance of ordinary shares (in shares)	1,570
Equity transaction costs		(523)			(523)
Stock-based compensation expense		1,821			1,821
Net loss				(21,707)	(21,707)
Stockholders' equity, ending balance at Jun. 30, 2019	$ 729	478,551	27	(435,902)	43,405
Stockholders' equity, ending balance (in shares) at Jun. 30, 2019	72,906
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 51	9,547			9,598
Issuance of ordinary shares (in shares)	5,088
Equity transaction costs		(131)			(131)
Stock-based compensation expense		4,138			4,138
Net loss				(17,795)	(17,795)
Stockholders' equity, ending balance at Sep. 30, 2019	$ 780	492,105	27	(453,697)	39,215
Stockholders' equity, ending balance (in shares) at Sep. 30, 2019	77,994
Stockholders' equity, beginning balance at Dec. 31, 2019	$ 945	517,044	27	(476,742)	41,274
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2019	94,545
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 5	181			186
Issuance of ordinary shares (in shares)	479
Shares issued in connection with the vesting of restricted stock units	$ 1	(1)
Shares issued in connection with the vesting of restricted stock units (in shares)	85
Equity transaction costs		(39)			(39)
Stock-based compensation expense		1,752			1,752
Net loss				(23,259)	(23,259)
Stockholders' equity, ending balance at Mar. 31, 2020	$ 951	518,937	27	(500,001)	19,914
Stockholders' equity, ending balance (in shares) at Mar. 31, 2020	95,109
Stockholders' equity, beginning balance at Dec. 31, 2019	$ 945	517,044	27	(476,742)	41,274
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2019	94,545
Consolidated Statements of Changes in Stockholders' Equity
Net loss					(51,651)
Stockholders' equity, ending balance at Sep. 30, 2020	$ 1,500	563,095	27	(528,393)	36,229
Stockholders' equity, ending balance (in shares) at Sep. 30, 2020	150,006
Stockholders' equity, beginning balance at Mar. 31, 2020	$ 951	518,937	27	(500,001)	19,914
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2020	95,109
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 476	40,891			41,367
Issuance of ordinary shares (in shares)	47,578
Shares issued in connection with the vesting of restricted stock units	$ 2	(2)
Shares issued in connection with the vesting of restricted stock units (in shares)	185
Shares issued in connection with the employee stock purchase plan	$ 1	42			43
Shares issued in connection with the employee stock purchase plan (in shares)	93
Equity transaction costs		(2,709)			(2,709)
Stock-based compensation expense		1,287			1,287
Net loss				(15,441)	(15,441)
Stockholders' equity, ending balance at Jun. 30, 2020	$ 1,430	558,446	27	(515,442)	44,461
Stockholders' equity, ending balance (in shares) at Jun. 30, 2020	142,965
Consolidated Statements of Changes in Stockholders' Equity
Issuance of ordinary shares	$ 69	4,036			4,105
Issuance of ordinary shares (in shares)	6,894
Shares issued in connection with the vesting of restricted stock units	$ 1	(1)
Shares issued in connection with the vesting of restricted stock units (in shares)	147
Equity transaction costs		(613)			(613)
Stock-based compensation expense		1,227			1,227
Net loss				(12,951)	(12,951)
Stockholders' equity, ending balance at Sep. 30, 2020	$ 1,500	$ 563,095	$ 27	$ (528,393)	$ 36,229
Stockholders' equity, ending balance (in shares) at Sep. 30, 2020	150,006