Pension and other postretirement benefits (net pension and postretirement benefit expense) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Postretirement Benefit Plan [Member]
Net pension and postretirement benefit expense [Abstract]
Interest cost	$ 6,669	$ 7,282	$ 8,560
Expected return on plan assets	(7,851)	(7,226)	(5,919)
Amortization of prior service credit	(3,743)	(3,742)	(3,815)
Amortization of net actuarial loss	5,415	5,406	8,383
Total periodic benefit expense	490	1,720	7,209
Settlement loss	0	0	0
Net periodic benefit expense	490	1,720	7,209
Pension Plans [Member]
Net pension and postretirement benefit expense [Abstract]
Interest cost	164	179	262
Expected return on plan assets	0	0	(57)
Amortization of prior service credit	0	0	0
Amortization of net actuarial loss	0	0	9
Total periodic benefit expense	164	179	214
Settlement loss	0	0	402
Net periodic benefit expense	$ 164	$ 179	$ 616