Share capital (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Feb. 03, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Common Stock Issued and Outstanding [Line Items]
Authorized Share Capital		571,428,571				571,428,571				571,428,571	571,428,571
Common shares, par value		$ 0.175				$ 0.175				$ 0.175	$ 0.175
Common Stock, Voting Rights										one vote per share
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									$ 2,704,406
Share repurchase program, cumulative shares repurchased										80,508,849
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 319,995								$ 319,995
Class of Warrant or Right, Outstanding		0								0
Warrants exercised										0	3,593,715
Cash dividends declared per share		$ 0.35	$ 0.35	$ 0.35	$ 0.35	0.32	$ 0.32	$ 0.32	$ 0.32	$ 1.40	$ 1.28	$ 1.20
Common shares
Schedule of Common Stock Issued and Outstanding [Line Items]
Cash dividends declared per share		0.35	0.35	$ 0.35	$ 0.35	$ 0.32	$ 0.32	$ 0.32	$ 0.32
Series A Preferred Stock
Schedule of Common Stock Issued and Outstanding [Line Items]
Cash dividends declared per share		$ 0.3671875	$ 0.3753472
Warrant [Member]
Schedule of Common Stock Issued and Outstanding [Line Items]
Number of warrants exercised during period										5,154,077
Warrants Expired During the Period										20,037