Earnings (loss) per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss)	$ 20,783	$ (58,273)	$ 12,775	$ (83,951)
Amount allocated to common stockholders (as percent)	92.06%	100.00%	92.04%	100.00%
Basic EPS:
Net income (loss) allocated to common stockholders	$ 19,132	$ (58,273)	$ 11,758	$ (83,951)
Net income (loss) allocated to common stockholders (shares)	87,318	87,076	87,282	87,059
Net income (loss) allocated to common stockholders, per share (in USD per share)	$ 0.22	$ (0.67)	$ 0.13	$ (0.96)
Effect of Dilutive Securities:
Share-based compensation	$ 0	$ 0	$ 0	$ 0
Share-based compensation (shares)	937	0	788	0
Diluted EPS:
Net income (loss) allocated to common stockholders with assumed conversion	$ 19,132	$ (58,273)	$ 11,758	$ (83,951)
Net income (loss) allocated to common stockholders with assumed conversion (shares)	88,255	87,076	88,070	87,059
Net income (loss) allocated to common stockholders with assumed conversion, per share (in USD per share)	$ 0.22	$ (0.67)	$ 0.13	$ (0.96)
Securities excluded from calculation of diluted EPS (shares)	535	1,822	524	1,548