Other commitments and contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Environmental matters [Line Items]
Accruals for environmental matters	$ 2,755	$ 2,646
Expense for environmental matters	750	348	$ (1,692)
Self-insurance
Self-insurance liabilities	6,627	$ 7,679
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