Share capital (Common stock) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Capital (Narrative)
Authorized share capital	571,428,571
Common shares, par value	$ 0.175
Common stock voting rights	one vote per share
Maximum voting percent	9.09%
Share repurchases
Share repurchase program, cumulative shares repurchased	81,035,969
Shares repurchased, cumulative	$ 2,730,975
Remaining share repurchase authorization amount	$ 293,426
Summary of common shares issued and outstanding
Common shares issued, beginning balance	161,994,491	161,279,976
Common shares issued, ending balance	162,003,969	161,285,411
Treasury shares, end of period	(82,674,941)	(82,147,821)
Common shares outstanding, ending balance	79,329,028	79,137,590
Restricted share awards
Summary of common shares issued and outstanding
Vested, net of shares withheld	7,423	3,440
Restricted share units
Summary of common shares issued and outstanding
Vested, net of shares withheld	2,055	1,995