Share capital (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended
	Feb. 05, 2014	Feb. 06, 2013	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Share Capital (Narrative)
Common shares, shares authorized			571,428,571		571,428,571
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
Stock Repurchase Program, Authorized Amount		$ 500,000
Shares repurchased, cumulative	1,774,436
Remaining share repurchase authorization amount			356,748
Deferred share units, adjustment factor			100.00%
Total outstanding deferred share units			0		0
Cash dividends declared per share			$ 0.3	$ 2.3
Summary of common shares issued and outstanding
Common shares issued, beginning balance			154,488,497	152,698,191
Options exercised (in shares)			90,019	128,020
Direct issuance of common stock			359	183
Common shares issued, ending balance			154,597,094	152,963,762
Shares repurchased, ending balance			(63,810,857)	(46,681,321)
Common shares outstanding, ending balance			90,786,237	106,282,441	96,044,312
Common shares
Share repurchases
Share repurchase program, cumulative shares repurchased			62,171,982
Shares repurchased, cumulative			$ 1,917,688
Cash dividends declared per share			$ 0.30	$ 0.30
Common shares | Special dividend declared
Share repurchases
Cash dividends declared per share				$ 2.00
Common share equivalents
Share repurchases
Cash dividends declared per share				$ 0.30
Common share equivalents | Special dividend declared
Share repurchases
Cash dividends declared per share				$ 2
Warrants
Share repurchases
Number of warrants exercised during period			0	0
Class of Warrant or Right, Outstanding			5,174,114		5,296,056
Deferred share units
Summary of common shares issued and outstanding
Deferred share units vested, net of shares withheld				2,935
Restricted share awards
Share repurchases
Vested (in shares)			27,353	148,304
Summary of common shares issued and outstanding
Vested, net of shares withheld			18,219	130,637
Restricted share units
Share repurchases
Vested (in shares)				5,468
Summary of common shares issued and outstanding
Vested, net of shares withheld				3,796
Employee director | Deferred share units
Share repurchases
Vested (in shares)			5,039
Summary of common shares issued and outstanding
Deferred share units vested, net of shares withheld			2,935