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Other commitments and contingencies (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Environmental matters [Line Items]      
Accruals for environmental matters $ 2,830   $ 3,206
Expense for environmental matters 175 $ (1,874)  
Self-insurance      
Self-insurance liabilities 6,931   $ 6,999
Environmental insurance policy purchased during 2002 [Member]      
Environmental matters [Line Items]      
Environmental insurance coverage 10,000    
Environmental insurance policy purchased during 2009 [Member]      
Environmental matters [Line Items]      
Environmental insurance coverage 15,000    
Environmental insurance policies in effect [Member]      
Environmental matters [Line Items]      
Accruals for environmental matters $ 0