Share capital (Summary of common shares issued and outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of common shares issued and outstanding
Common shares issued, beginning balance	160,570,772	155,554,224	154,488,497
Options exercised	30,530	782,465	412,656
Warrants exercised	0	3,593,715
Direct issuance of common stock	0	639	1,380
Common shares issued, ending balance	161,279,976	160,570,772	155,554,224
Treasury shares, ending balance	(82,147,724)	(77,670,155)	(71,684,379)
Common shares outstanding, ending balance	79,132,252	82,900,617	83,869,845
Restricted Stock [Member]
Summary of common shares issued and outstanding
Vested, net of shares withheld	612,100	614,945	615,659
Restricted share units [Member]
Summary of common shares issued and outstanding
Vested, net of shares withheld	18,486	13,260	10,265
Performance Shares [Member]
Summary of common shares issued and outstanding
Vested, net of shares withheld	48,088	11,524	25,767