Net loss per share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (11,886)	$ (5,362)	$ (43,755)	$ (81,714)
Denominator:
Weighted average common shares outstanding, basic and diluted	28,499,475	168,071
Net loss per share, basic and diluted	$ (0.42)	$ (31.90)
Number of weighted average restricted common stock	77,223	70,915