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Earnings (loss) per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income (loss)	$ (16,163)	$ 73,779
Amount allocated to common stockholders (in hundredths)	100.00%	91.91%
Basic EPS:
Net income (loss) allocated to common stockholders	$ (16,163)	$ 67,813
Net income (loss) allocated to common stockholders - (shares)	87,040,000	88,814,000
Net income (loss) allocated to common stockholders (in dollars per share)	$ (0.19)	$ 0.76
Effect of Dilutive Securities:
Share-based compensation plans	$ 0	$ 0
Share-based compensation plans - (shares)	0	555,000
Diluted EPS:
Net income (loss) allocated to common stockholders with assumed conversion	$ (16,163)	$ 67,813
Net income (loss) allocated to common stockholders with assumed conversion - (shares)	87,040,000	89,369,000
Net income (loss) allocated to common stockholders with assumed conversion (in dollars per share)	$ (0.19)	$ 0.76
Stock options [Member]
Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted EPS (shares)	393,237	333,266
Service-based share awards [Member]
Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted EPS (shares)	498,809	0

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