Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities	$ 47,018	$ 45,821
Additions to properties, plants, and equipment	(43,304)	(45,964)
Other investing activities, net	28,117	(8,782)
Dividends paid to stockholders	(2,276)	(2,257)
Issuance of debt	31,024
Payments on debt	(3,762)	(3,715)
Other financing activity	(2,697)	7,045
Effect of exchange rate changes on cash	(532)	1,086
Changes in cash, cash equivalents and restricted cash and cash equivalents	53,588	(6,766)
Beginning cash, cash equivalents and restricted cash and cash equivalents	186,107	171,977
Ending cash, cash equivalents and restricted cash and cash equivalents	239,722	165,211
Additions to properties, plants, equipment and mineral interests	(43,304)	(45,964)
Payments on debt	(3,762)	(3,715)
Reportable Legal Entities [Member] | Parent Company [Member]
Cash flows from operating activities	27,981	9,131
Additions to properties, plants, and equipment
Other investing activities, net	6,294	(8,500)
Dividends paid to stockholders	(2,276)	(2,257)
Issuance of debt	31,024
Payments on debt
Other financing activity	(3,544)	(11,169)
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents	59,479	(12,795)
Beginning cash, cash equivalents and restricted cash and cash equivalents	103,878	113,275
Ending cash, cash equivalents and restricted cash and cash equivalents	163,357	100,480
Additions to properties, plants, equipment and mineral interests
Payments on debt
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash flows from operating activities	60,805	13,792
Additions to properties, plants, and equipment	(25,454)	(19,132)
Other investing activities, net	44	164
Dividends paid to stockholders
Issuance of debt
Payments on debt	(1,393)	(3,044)
Other financing activity	(40,658)	9,501
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents	(6,656)	1,281
Beginning cash, cash equivalents and restricted cash and cash equivalents	31,016	26,588
Ending cash, cash equivalents and restricted cash and cash equivalents	24,387	27,869
Additions to properties, plants, equipment and mineral interests	(25,454)	(19,132)
Payments on debt	(1,393)	(3,044)
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities	2,166	10,757
Additions to properties, plants, and equipment	(17,850)	(26,832)
Other investing activities, net	420	(22)
Dividends paid to stockholders
Issuance of debt
Payments on debt	(2,369)	(671)
Other financing activity	18,930	20,430
Effect of exchange rate changes on cash	(532)	1,086
Changes in cash, cash equivalents and restricted cash and cash equivalents	765	4,748
Beginning cash, cash equivalents and restricted cash and cash equivalents	51,213	32,114
Ending cash, cash equivalents and restricted cash and cash equivalents	51,978	36,862
Additions to properties, plants, equipment and mineral interests	(17,850)	(26,832)
Payments on debt	(2,369)	(671)
Consolidation, Eliminations [Member]
Cash flows from operating activities	(43,934)	12,141
Additions to properties, plants, and equipment
Other investing activities, net	21,359	(424)
Dividends paid to stockholders
Issuance of debt
Payments on debt
Other financing activity	22,575	(11,717)
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents
Beginning cash, cash equivalents and restricted cash and cash equivalents
Ending cash, cash equivalents and restricted cash and cash equivalents
Additions to properties, plants, equipment and mineral interests
Payments on debt