Other commitments and contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Environmental matters [Line Items]
Accruals for environmental matters	$ 3,206	$ 5,952
Expense for environmental matters	(1,692)	1,142	$ 1,079
Self-insurance
Self-insurance liabilities	6,999	$ 6,457
Litigation
Damages sought	43,000
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