Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended									9 Months Ended		12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 21, 2017	Dec. 31, 2017		Dec. 31, 2019		Dec. 31, 2018
Numerator for basic and diluted earnings per share
Net (loss) income		$ (189,244)	$ (130,935)	$ (45,229)	$ (103,540)	$ 78,945	$ (12,068)	$ (21,993)	$ (11,442)	$ 1,041,959	$ (118,902)		$ (468,948)		$ 33,442
Denominator for basic earnings per share
Weighted average common shares - Basic for Class A and Class B (in shares)	[1]										44,984,046		45,637,338		45,288,980
Effect of dilutive securities
Dilutive shares from equity compensation awards (in shares)											0		0		441,191
Denominator for diluted earnings per share
Weighted average common shares - Diluted for Class A and Class B (in shares)	[1]										44,984,046		45,637,338		45,730,171
Earnings (loss) per share
Basic for Class A and Class B (in dollars per share)		$ (4.14)	$ (2.86)	$ (0.99)	$ (2.28)	$ 1.74	$ (0.27)	$ (0.49)	$ (0.25)		$ (2.64)	[1]	$ (10.28)	[1]	$ 0.74	[1]
Diluted for Class A and Class B (in dollars per share)		$ (4.14)	$ (2.86)	$ (0.99)	$ (2.28)	$ 1.73	$ (0.27)	$ (0.49)	$ (0.25)		$ (2.64)	[1]	$ (10.28)	[1]	$ 0.73	[1]
Unvested Restricted Stock Awards
Securities excluded from earnings per share calculations
Unvested restricted stock awards or units at period end (in shares)											1,833,136		3,187,231		125,323
Warrants
Securities excluded from earnings per share calculations
Warrants (in shares)											140,023		0		0

[1] See “Note 2: Earnings per share.”