Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Shares issued, Beginning of year	479,783,864	342,832,412	342,832,412
Stock Issued During Period, Shares, New Issues	136,951,452	0	0
Common Shares in treasury, end of year	(13,815,148)	(18,268,971)	(14,172,726)
Shares issued and outstanding, end of year	465,968,716	324,563,441	328,659,686
Common Stock Issued to Employee Trust, beginning of year	(5,800)	(9,467)	(9,467)
Shares redeemed	3,000	3,667	0
Common Stock Issued to Employee Trust, end of year	(2,800)	(5,800)	(9,467)