Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes In Common Stock Shares Issues And Outstanding [Roll Forward]
Shares issued, beginning of year	342,832,412	341,094,559	337,841,616
Shares issued, net	0	0	2,268,000
Exercise of stock options	0	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
Balance, beginning of year	(9,467)	(101,481)	(101,481)
Shares redeemed	0	92,014	0
Balance, end of year	(9,467)	(9,467)	(101,481)