Loss per common share - Summary of computation of basic and diluted loss per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net loss	$ (42,652)	$ (39,383)	$ (51,976)	$ (56,849)	$ (82,035)	$ (108,825)
Denominator:
Weighted average common shares for basic loss per common share	394,074		271,876		370,776	271,668
Effect of dilutive securities	0		0		0	0
Weighted average common shares for diluted loss per common share	394,074		271,876		370,776	271,668
Loss per common share, basic and diluted	$ (0.11)		$ (0.19)		$ (0.22)	$ (0.40)