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Earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss)	$ 35,805	$ 20,783	$ 7,131	$ (15,139)	$ (168,464)	$ (58,273)	$ (9,515)	$ (16,163)	$ 48,580	$ (252,415)	$ (59,310)
Percentage allocated to common shareholders									92.00%	100.00%	100.00%
Basic EPS:
Net income allocated to common stockholders, basic	$ 32,963	$ 19,132	$ 6,563	$ (15,139)	$ (168,464)	$ (58,273)	$ (9,515)	$ (16,163)	$ 44,714
Shares outstanding, basic (in shares)									87,295
Basic (in dollars per share)	$ 0.38	$ 0.22	$ 0.08	$ (0.17)	$ (1.93)	$ (0.67)	$ (0.11)	$ (0.19)	$ 0.51	$ (2.90)	$ (0.68)
Diluted EPS:
Net income allocated to common stockholders, diluted									$ 44,714
Shares outstanding, diluted (in shares)									88,020
Diluted (in dollars per share)	$ 0.37	$ 0.22	$ 0.07	$ (0.17)	$ (1.93)	$ (0.67)	$ (0.11)	$ (0.19)	$ 0.51	$ (2.90)	$ (0.68)
Basic and Diluted EPS:
Net income (loss) allocated to common stockholders, basic and diluted										$ (252,415)	$ (59,310)
Shares outstanding, basic and diluted (in shares)										87,064	87,375
Basic and diluted (in dollars per share)										$ (2.90)	$ (0.68)
Employee Stock Option [Member]
Diluted EPS:
Dilutive effect of share-based payment arrangements									101
Service-Based Share Awards [Member]
Diluted EPS:
Dilutive effect of share-based payment arrangements									624

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