Other commitments and contingencies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Environmental matters [Line Items]
Accruals for environmental matters	$ 3,140		$ 3,206
Expense for environmental matters	84	$ 171
Self-insurance
Self-insurance liabilities	7,592		$ 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