Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities
Net income	$ 91,547,000	$ 86,198,000	$ 76,037,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	79,022,000	74,967,000	66,985,000
Deferred compensation	4,857,000	16,805,000	6,500,000
Realized and unrealized gain on long-term investments	(4,178,000)	(152,000)	(21,000)
Stock-based compensation	7,151,000	12,618,000	4,374,000
Deferred income taxes	550,000	(8,426,000)	3,001,000
Other non-cash items	4,857,000	7,334,000	3,830,000
Changes in assets and liabilities, excluding effects of acquisitions:
Receivables, net	(16,358,000)	(17,180,000)	4,160,000
Inventories	5,655,000	(3,774,000)	2,851,000
Other current assets	(489,000)	1,471,000	(3,410,000)
Accounts payable and accrued liabilities	30,476,000	38,261,000	21,219,000
Pension liabilities	(1,960,000)	607,000	932,000
Environmental and legal liabilities	(1,142,000)	4,561,000	(1,398,000)
Deferred revenues	(1,125,000)	(1,128,000)	(1,345,000)
Net Cash Provided By Operating Activities	198,863,000	212,162,000	183,715,000
Cash Flows From Investing Activities
Expenditures for property, plant and equipment	(78,613,000)	(103,076,000)	(119,349,000)
Business acquisitions, net of cash acquired (Note 20)	(4,339,000)	(23,510,000)	(5,133,000)
Proceeds from sale of product line (Note 21)			3,262,000
Other, net	269,000	(3,935,000)	(4,750,000)
Net Cash Used In Investing Activities	(82,683,000)	(130,521,000)	(125,970,000)
Cash Flows From Financing Activities
Revolving debt and bank overdrafts, net	(6,008,000)	1,292,000	(26,217,000)
Other debt borrowings			100,000,000
Other debt repayments	(20,714,000)	(15,069,000)	(13,098,000)
Dividends paid	(18,907,000)	(17,329,000)	(16,300,000)
Company stock repurchased	(6,000,000)	(2,408,000)	(2,000,000)
Stock option exercises	3,370,000	4,017,000	777,000
Other, net	(2,238,000)	(275,000)	(673,000)
Net Cash Provided By (Used In) Financing Activities	(50,497,000)	(29,772,000)	42,489,000
Effect of Exchange Rate Changes on Cash	7,468,000	(2,269,000)	(9,306,000)
Net Increase in Cash and Cash Equivalents	73,151,000	49,600,000	90,928,000
Cash and Cash Equivalents at Beginning of Year	225,743,000	176,143,000	85,215,000
Cash and Cash Equivalents at End of Year	298,894,000	225,743,000	176,143,000
Supplemental Cash Flow Information
Cash payments of income taxes, net of refunds	25,661,000	30,581,000	21,784,000
Cash payments of interest	$ 13,889,000	$ 14,730,000	$ 11,943,000