Shareholders' equity (accumulated other comprehensive loss) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Postretirement and defined benefit pension plans:
Unrealized prior service credit	$ 13,321	$ 15,651	$ 17,978
Unrealized net actuarial losses	(75,599)	(68,343)	(70,328)
Postretirement and defined benefit pension plans, net of tax	(62,278)	(52,692)	(52,350)
Loss on derivatives, net of tax	(2,931)	(4,522)	(5,841)	[1]
Net unrealized gain on marketable securities, net of tax	178	13	0
Currency translation adjustment	5,630	7,189	5,373
Accumulated other comprehensive loss	$ (59,401)	$ (50,012)	$ (52,818)

[1] Relates to interest rate locks executed in 2004 and 2002. see Note 6 for further information regarding these financial instruments