Share capital (Summary of common shares issued and outstanding) (Details) - USD ($) $ in Thousands		12 Months Ended
	Feb. 03, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Common Stock Issued and Outstanding [Line Items]
Common Stock, Voting Rights		one vote per share
Summary of common shares issued and outstanding
Common shares issued, beginning balance		161,279,976	160,570,772	155,554,224
Options exercised		26,136	30,530	782,465
Warrants exercised		0	0	3,593,715
Direct issuance of common stock		0	0	639
Common shares issued, ending balance		161,994,491	161,279,976	160,570,772
Treasury shares, ending balance		(82,674,941)	(82,147,724)	(77,670,155)
Common shares outstanding, ending balance		79,319,550	79,132,252	82,900,617
Maximum Percent Of Outstanding Common Shares Controlled Contingent Under Voting Rights		9.09%
Stock repurchase program, authorized amount	$ 750,000
Share repurchase program cumulative aggregrate purchase value	$ 2,274,401	$ 2,730,975
Share repurchase program, cumulative shares repurchased		81,035,969
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 293,426
Restricted Stock
Summary of common shares issued and outstanding
Vested, net of shares withheld		630,515	612,100	614,945
Restricted share units
Summary of common shares issued and outstanding
Vested, net of shares withheld		15,454	18,486	13,260
Performance Shares
Summary of common shares issued and outstanding
Vested, net of shares withheld		42,410	48,088	11,524