Other commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Environmental matters [Line Items]
Accruals for environmental matters	$ 8,294	$ 8,446
Expense for environmental matters	1,169	969	353
Self-insurance
Workers' compensation liability	4,560	4,471
Liability for medical and dental benefits for active and disabled employees	3,322	3,872
Liability for medical and dental benefits for active and disabled employees, difference between discounted and undiscounted amount	149	146
Environmental insurance policy purchased during 2002 | Environmental remediation costs
Environmental matters [Line Items]
Accruals for environmental matters	4,173
Environmental insurance coverage	12,911
Cumulative benefits received under insurance policy	8,738
Environmental insurance policy purchased during 2002 | Third-party claims
Environmental matters [Line Items]
Environmental insurance coverage	10,000
Environmental insurance policy for facilities purchased subsequent to 2002
Environmental matters [Line Items]
Environmental insurance coverage	$ 15,000