Significant acquisitions - Assets Acquired Liabilities Assumed (Details) - USD ($)	May 26, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets acquired and liabilities assumed:
Goodwill		$ 120,220,000	$ 113,343,000	$ 113,996,000
Cap Juluca
Consideration:
Agreed cash consideration	$ 70,759,000
Less: Working capital adjustment	(2,107,000)
Payments to Acquire Businesses, Net of Cash Acquired	68,652,000
Assets acquired and liabilities assumed:
Cash and cash equivalents	20,000
Accounts receivable	112,000
Prepaid expenses and other	45,000
Inventories	108,000
Property, plant and equipment	59,159,000
Other intangible assets	6,100,000
Accounts payable	(595,000)
Accrued liabilities	(360,000)
Deferred revenue	(1,437,000)
Goodwill	5,500,000	$ 5,500,000
Net assets acquired	$ 68,652,000