Share capital (Common stock) (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Feb. 03, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Common Stock Issued and Outstanding [Line Items]
Authorized share capital		571,428,571
Share Capital (Narrative)
Common shares, par value		$ 0.175
Common stock voting rights		one vote per share
Restriction on percentage of voting power		9.09%
Share repurchases
Common share repurchase authorization	$ 750,000
Share repurchase program, cumulative shares repurchased		80,191,448
Shares repurchased, cumulative	$ 2,274,401	$ 2,687,746
Remaining share repurchase authorization amount		$ 336,655
Summary of common shares issued and outstanding
Common shares issued, beginning balance		160,570,772	155,554,224
Options exercised (in shares)		27,983	782,465
Warrants exercised (in shares)		0	1,461,715
Common shares issued, ending balance		161,273,353	158,434,541
Treasury shares, ending balance		(81,830,323)	(76,436,650)
Common shares outstanding, ending balance		79,443,030	81,997,891
Common shares
Summary of common shares issued and outstanding
Direct issuance of common stock (in shares)		0	639
Performance shares
Summary of common shares issued and outstanding
Vested, net of shares withheld		48,088	11,524
Restricted share awards
Summary of common shares issued and outstanding
Vested, net of shares withheld		608,024	610,714
Restricted share units
Summary of common shares issued and outstanding
Vested, net of shares withheld		18,486	13,260