Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Total
Balance at the beginning of period at Dec. 31, 2017	$ 3	$ 123,879	$ (169)	$ (41,561)	$ 82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		172			172
Foreign currency translation			3		3
Net loss				(7,483)	(7,483)
Balance at the end of period at Mar. 31, 2018	$ 3	124,051	(166)	(49,044)	74,844
Balance at the end of period (in shares) at Mar. 31, 2018	26,749,666
Balance at the beginning of period at Dec. 31, 2017	$ 3	123,879	(169)	(41,561)	82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Increase (Decrease) in Stockholders' Equity
Net loss					(29,214)
Balance at the end of period at Sep. 30, 2018	$ 3	225,848	(48)	(70,775)	155,028
Balance at the end of period (in shares) at Sep. 30, 2018	34,193,666
Balance at the beginning of period at Dec. 31, 2017	$ 3	123,879	(169)	(41,561)	82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Balance at the end of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the end of period (in shares) at Dec. 31, 2018	34,193,666
Balance at the beginning of period at Mar. 31, 2018	$ 3	124,051	(166)	(49,044)	74,844
Balance at the beginning of period (in shares) at Mar. 31, 2018	26,749,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		904			904
Foreign currency translation			78		78
Net loss				(10,305)	(10,305)
Balance at the end of period at Jun. 30, 2018	$ 3	124,955	(88)	(59,349)	65,521
Balance at the end of period (in shares) at Jun. 30, 2018	26,749,666
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in initial public offering, net of issuance costs		100,498			100,498
Issuance of common stock (in shares)	6,900,000
Issuance of common stock to nonemployees (in shares)	544,000
Stock-based compensation expense		395			395
Foreign currency translation			40		40
Net loss				(11,426)	(11,426)
Balance at the end of period at Sep. 30, 2018	$ 3	225,848	(48)	(70,775)	155,028
Balance at the end of period (in shares) at Sep. 30, 2018	34,193,666
Balance at the beginning of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the beginning of period (in shares) at Dec. 31, 2018	34,193,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		864			864
Foreign currency translation			56		56
Net loss				(15,934)	(15,934)
Balance at the end of period at Mar. 31, 2019	$ 3	226,216	63	(100,769)	125,513
Balance at the end of period (in shares) at Mar. 31, 2019	34,193,666
Balance at the beginning of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the beginning of period (in shares) at Dec. 31, 2018	34,193,666
Increase (Decrease) in Stockholders' Equity
Net loss					(57,892)
Balance at the end of period at Sep. 30, 2019	$ 3	228,532	131	(142,727)	85,939
Balance at the end of period (in shares) at Sep. 30, 2019	34,593,666
Balance at the beginning of period at Mar. 31, 2019	$ 3	226,216	63	(100,769)	125,513
Balance at the beginning of period (in shares) at Mar. 31, 2019	34,193,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		971			971
Foreign currency translation			(66)		(66)
Net loss				(18,036)	(18,036)
Balance at the end of period at Jun. 30, 2019	$ 3	227,187	(3)	(118,805)	108,382
Balance at the end of period (in shares) at Jun. 30, 2019	34,193,666
Increase (Decrease) in Stockholders' Equity
Issuance of common stock to nonemployees (in shares)	400,000
Stock-based compensation expense		1,345			1,345
Foreign currency translation			134		134
Net loss				(23,922)	(23,922)
Balance at the end of period at Sep. 30, 2019	$ 3	$ 228,532	$ 131	$ (142,727)	$ 85,939
Balance at the end of period (in shares) at Sep. 30, 2019	34,593,666