Earnings (loss)per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Weighted-average number of common shares outstanding:
Basic number of common shares outstanding, net of treasury shares	102,283,844	104,132,797	103,144,078	104,195,336
Dilutive effect of options	1,084,217	403,547	937,864	358,015
Dilutive effect of warrants	988,830	556,829	860,484	553,222
Dilutive effect of restricted shares with service and performance condition	212,335	140,748	207,284	121,601
Diluted number of common shares outstanding	104,569,226	105,233,921	105,149,710	105,228,174
Basic earnings per common share:
Net income	$ 74,578	$ 53,376	$ 178,764	$ 2,247
Income allocated to participating shares	(71)	(169)	(204)	(7)
Net income available to common shareholders	$ 74,507	$ 53,207	$ 178,560	$ 2,240
Basic earnings per common share	$ 0.73	$ 0.51	$ 1.73	$ 0.02
Earnings per share, diluted [Abstract]
Net income	$ 74,578	$ 53,376	$ 178,764	$ 2,247
Income allocated to participating shares	(69)	(167)	200	7
Net income available to common shareholders	$ 74,509	$ 53,209	$ 178,564	$ 2,240
Diluted earnings per common share	$ 0.71	$ 0.51	$ 1.70	$ 0.02
Options
Earnings per share, diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share	4,322,659	4,370,193	4,322,659	4,416,193