Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Weighted-average number of common shares outstanding:
Basic number of common shares outstanding	104,117,448	103,295,920	103,931,871	103,275,204
Dilutive effect of options	0	0	0	1,494,711
Dilutive effect of warrants	0	0	0	1,684,860
Diluted number of common shares outstanding	104,117,448	103,295,920	103,931,871	106,454,775
Basic net income (loss) per common share:
Net income (loss)	$ (195,715)	$ (5,997)	$ (129,583)	$ 65,074
Income allocated to participating shares	0	0	0	(420)
Net income (loss) available to common shareholders	$ (195,715)	$ (5,997)	$ (129,583)	$ 64,654
Basic net income (loss) per common share	$ (1.88)	$ (0.06)	$ (1.25)	$ 0.63
Earnings per share, diluted [Abstract]
Net income (loss)	$ (195,715)	$ (5,997)	$ (129,583)	$ 65,074
Income allocated to participating securities	0	0	0	(408)
Net income (loss) available to common shareholders	$ (195,715)	$ (5,997)	$ (129,583)	$ 64,666
Diluted net income (loss) per common share	$ (1.88)	$ (0.06)	$ (1.25)	$ 0.61
Options
Earnings per share, diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share	10,350,458	11,327,302	9,863,064	4,553,159
Warrant [Member]
Earnings per share, diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share	4,651,163	4,651,163	4,651,163