Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Shares issued, Beginning of year	479,783,864	479,783,864	479,783,864
Treasury Stock, Shares, Retired	(2,178,600)	0	0
Shares issued, End of year	477,605,264	479,783,864	479,783,864
Common Shares in treasury, end of year	(26,872,639)	(27,812,297)	(20,580,486)	(15,950,685)
Stock Issued During Period, Shares, New Issues	2,345,208	3,210,427	3,089,234
Treasury Stock, Shares, Acquired	(3,584,150)	(10,442,238)	(7,719,035)
Shares issued and outstanding, end of year	450,732,625	451,971,567	459,203,378