Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator for basic and diluted loss per share
Net loss	$ (438,726)	$ 4,917	$ (45,229)	$ (103,540)	$ (433,809)	$ (148,769)
Denominator for basic loss per share
Weighted average common shares - Basic for Class A and Class B (in shares)	45,949,797		45,641,797		45,890,041	45,549,518
Denominator for diluted loss per share
Weighted average common shares - Diluted for Class A and Class B (in shares)	45,949,797		45,641,797		45,890,041	45,549,518
Loss per share
Basic for Class A and Class B (in dollars per share)	$ (9.55)		$ (0.99)		$ (9.45)	$ (3.27)
Diluted for Class A and Class B (in dollars per share)	$ (9.55)		$ (0.99)		$ (9.45)	$ (3.27)
Unvested restricted stock units - stock settled
Loss per share
Participating securities excluded from earnings per share calculations (in shares)	604,789		81,119		604,789	81,119
Unvested restricted stock awards
Loss per share
Participating securities excluded from earnings per share calculations (in shares)	1,839,381		706,821		1,839,381	706,821