Pension and other postretirement benefits (obligations and funded status) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Apr. 30, 2009	Dec. 31, 2008
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Amount recognized in accumulated other comprehensive loss, net of tax	$ (62,278)	$ (52,692)	$ (52,350)
Postretirement Benefit Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of period	141,341	138,915
Interest cost	6,669	7,282	8,560
Actuarial loss - net	8,199	5,781
Benefits paid from the VEBA trust (see Note 11) and company funds	(10,940)	(11,363)
Medicare Part D reimbursements	856	726
Benefit obligation, end of period	146,125	141,341	138,915
Change in plan assets [Roll Forward]
Fair value of plan assets, beginning of period	101,310	90,320
Actual (loss) gain on plan assets	(688)	10,990
Fair value of plan assets, end of period	100,622	101,310	90,320
Funded status	(45,503)	(40,031)		(55,928)	(60,437)
Employees who retired after 1986 that are receiving medical benefits as a percentage of total retirees receiving medical benefits (in hundredths)	80.00%
Amounts recognized in Balance Sheet [Abstract]
Accrued liabilities	0	0
Other non-current liabilities	45,503	40,031
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Unrecognized prior service credit	(20,697)	(24,440)
Unrecognized net actuarial loss	119,681	108,358
Tax effect	(37,021)	(31,334)
Amount recognized in accumulated other comprehensive loss, net of tax	61,963	52,584
Weighted-average amortization period, prior service credit (in years)	16
Average remaining life expectancy of inactive plan participants (in years)	17.9			18.8
Amounts recognized in accumulated comprehensive loss which are expected to be recognized in postretirement benefit expense in the next 12 months [Abstract]
Prior service credit	(3,055)
Net actuarial loss	5,870
Total	2,815
Pension Plans [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of period	3,508	3,455
Interest cost	164	179	262
Actuarial loss - net	332	198
Benefits paid from the VEBA trust (see Note 11) and company funds	(324)	(324)
Medicare Part D reimbursements	0	0
Benefit obligation, end of period	3,680	3,508	3,455
Change in plan assets [Roll Forward]
Fair value of plan assets, beginning of period	0	0
Actual (loss) gain on plan assets	0	0
Fair value of plan assets, end of period	0	0	0
Funded status	(3,680)	(3,508)
Amounts recognized in Balance Sheet [Abstract]
Accrued liabilities	324	324
Other non-current liabilities	3,356	3,184
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Unrecognized prior service credit	0	0
Unrecognized net actuarial loss	493	161
Tax effect	(178)	(53)
Amount recognized in accumulated other comprehensive loss, net of tax	315	108
Amounts recognized in accumulated comprehensive loss which are expected to be recognized in postretirement benefit expense in the next 12 months [Abstract]
Prior service credit	0
Net actuarial loss	9
Total	$ 9