Other commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Environmental matters [Abstract]
Accruals for environmental matters	$ 8,730	$ 9,293
Environmental remediation costs	353	868	1,327
Portion of accruals covered by environmental insurance policy	6,440
Maximum losses covered for remediation costs under 2002 environmental insurance policy	12,911
Cumulative losses for which payment has been received under 2002 environmental insurance policy	6,471
Maximum losses covered for third-party claims under 2002 environmental insurance policy	10,000
Maximum losses covered for remediation costs related to facilities acquired after 2002	15,000
Self-insurance [Abstract]
Workers' compensation liability	5,141	4,716
Difference between discounted and undiscounted amount of workers' compensation liability	20	29
Liability for medical and dental benefits for active and disabled employees	3,848	4,167
Difference between discounted and undiscounted amount of liability for medical and dental benefits for active and disabled employees	$ 296	$ 419