Shareholders' equity (Rollforward Of Changes In Common Stock Shares Issued And Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stockholders' Equity Note [Abstract]
Shares issued, beginning of year	342,832,412	342,832,412
Shares issued, end of year	342,832,412	342,832,412	342,832,412
Common Shares in treasury, end of year	(18,268,971)	(14,172,726)	(3,038,477)
Shares issued and outstanding, end of year	324,563,441	328,659,686	339,793,935
Balance, beginning of year	(9,467)	(9,467)	(9,467)
Shares redeemed	3,667	0	0
Balance, end of year	(5,800)	(9,467)	(9,467)