Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator for basic and diluted earnings per share
Net loss		$ (45,229)	$ (103,540)	$ (21,993)	$ (11,442)	$ (148,769)	$ (33,435)
Denominator for basic earnings per share
Weighted average common shares - Basic for Class A and Class B (in shares)	[1]	45,641,797		45,338,650		45,549,518	45,241,513
Denominator for diluted earnings per share
Weighted average common shares - Diluted for Class A and Class B (in shares)	[1]	45,641,797		45,338,650		45,549,518	45,241,513
Earnings per share
Basic for Class A and Class B (in dollars per share)	[1]	$ (0.99)		$ (0.49)		$ (3.27)	$ (0.74)
Diluted for Class A and Class B (in dollars per share)	[1]	$ (0.99)		$ (0.49)		$ (3.27)	$ (0.74)
Warrants
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)		0		140,023		0	140,023
Unvested restricted stock units - stock settled
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)		81,119		0		81,119	0
Unvested restricted stock awards
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)		706,821		1,126,669		706,821	1,126,669

[1] See “Note 2: Earnings per share.”