Share capital (Common stock) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended				6 Months Ended
	Feb. 03, 2015	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Common Stock Issued and Outstanding [Line Items]
Authorized share capital		571,428,571				571,428,571
Share Capital (Narrative)
Common shares, par value		$ 0.175				$ 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						78,841,758
Shares repurchased, cumulative	$ 2,274,401					$ 2,620,814
Remaining share repurchase authorization amount		$ 403,587				$ 403,587
Cash dividends declared per share		$ 0.35		$ 0.32		$ 0.7	$ 0.64
Summary of common shares issued and outstanding
Common shares issued, beginning balance			160,570,772		155,554,224	160,570,772	155,554,224
Options exercised (in shares)						13,635	728,489
Warrants exercised (in shares)							1,461,715
Common shares issued, ending balance		161,252,871		158,379,505		161,252,871	158,379,505
Treasury shares, ending balance		(80,480,633)		(75,083,710)		(80,480,633)	(75,083,710)
Common shares outstanding, ending balance		80,772,238		83,295,795		80,772,238	83,295,795
Common shares
Share repurchases
Cash dividends declared per share		$ 0.35	$ 0.35	$ 0.32	$ 0.32
Summary of common shares issued and outstanding
Direct issuance of common stock (in shares)							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						601,890	609,654
Restricted share units
Summary of common shares issued and outstanding
Vested, net of shares withheld						18,486	13,260