Shareholders' equity (Tables)

12 Months Ended

Dec. 31, 2012

Stockholders' Equity Note [Abstract]

Schedule of changes in Common Shares issued and outstanding

The following table presents a roll-forward of changes in Common Shares issued and outstanding:

	Years Ended December 31
	2012		2011		2010
Shares issued, beginning of year	342,832,412		341,094,559		337,841,616
Shares issued, net	—		—		2,268,000
Exercise of stock options	—		1,737,853		984,943
Shares issued, end of year	342,832,412		342,832,412		341,094,559
Common Shares in treasury, end of year	(2,510,878	)	(5,905,136	)	(6,151,707	)
Shares issued and outstanding, end of year	340,321,534		336,927,276		334,942,852
Common Shares issued to employee trust
Balance, beginning of year	(9,467	)	(101,481	)	(101,481	)
Shares redeemed	—		92,014		—
Balance, end of year	(9,467	)	(9,467	)	(101,481	)