Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended						9 Months Ended
		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator for basic and diluted earnings per share
Net loss		$ (130,935)	$ (45,229)	$ (103,540)	$ (12,068)	$ (21,993)	$ (11,442)	$ (279,704)	$ (45,503)
Denominator for basic earnings per share
Weighted average common shares - Basic for Class A and Class B (in shares)	[1]	45,716,522			45,333,745			45,605,798	45,272,595
Denominator for diluted earnings per share
Weighted average common shares - Diluted for Class A and Class B (in shares)	[1]	45,716,522			45,333,745			45,605,798	45,272,595
Earnings per share
Basic for Class A and Class B (in dollars per share)	[1]	$ (2.86)			$ (0.27)			$ (6.13)	$ (1.01)
Diluted for Class A and Class B (in dollars per share)	[1]	$ (2.86)			$ (0.27)			$ (6.13)	$ (1.01)
Unvested restricted stock units - stock settled
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)		838,552			0			838,552	0
Unvested restricted stock awards
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)		680,152			1,126,669			680,152	1,126,669

[1] See “Note 2: Earnings per share.”