Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares issued, Beginning of year	479,783,864	479,783,864	479,783,864
Common Shares in treasury, end of year	(27,812,297)	(20,580,486)	(15,950,685)	(13,815,148)
Stock Issued During Period, Shares, New Issues	3,210,427	3,089,234	3,731,075
Treasury Stock, Shares, Acquired	(10,442,238)	(7,719,035)	(5,866,612)
Shares issued, End of year	479,783,864	479,783,864	479,783,864
Shares issued and outstanding, end of year	451,971,567	459,203,378	463,833,179