Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Shares issued, Beginning of year	479,783,864	479,783,864	342,832,412	342,832,412
Stock Issued During Period, Shares, New Issues	0	136,951,452	0
Common Shares in treasury, end of year	(15,950,685)	(13,815,148)	(18,268,971)
Shares issued and outstanding, end of year	463,833,179	465,968,716	324,563,441