Shareholders' equity (Tables)

12 Months Ended

Dec. 31, 2013

Stockholders' Equity Note [Abstract]

Schedule of changes in Common Shares issued and outstanding

	Years Ended December 31
	2013		2012		2011
Shares issued, beginning of year	342,832,412		342,832,412		341,094,559
Exercise of stock options	—		—		1,737,853
Shares issued, end of year	342,832,412		342,832,412		342,832,412
Common Shares in treasury, end of year (at cost)	(3,038,477	)	(2,510,878	)	(5,905,136	)
Shares issued and outstanding, end of year	339,793,935		340,321,534		336,927,276
Common Shares issued to employee trust
Balance, beginning of year	(9,467	)	(9,467	)	(101,481	)
Shares redeemed	—		—		92,014
Balance, end of year	(9,467	)	(9,467	)	(9,467	)