Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities	$ 16,383	$ 38,285
Additions to properties, plants, equipment and mineral interests	(17,635)	(21,658)
Other investing activities, net	(530)	(7,418)
Dividends paid to stockholders	(1,136)	(1,127)
Borrowings on debt	31,024
Payments on debt	(1,322)	(2,065)
Other financing activity	(1,225)	(822)
Effect of exchange rate changes on cash	876	1,814
Changes in cash, cash equivalents and restricted cash and cash equivalents	26,435	7,009
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	187,139	171,977
Cash, cash equivalents and restricted cash and cash equivalents at end of period	213,574	178,986
Consolidation, Eliminations [Member]
Cash flows from operating activities	(36,943)	(29,818)
Additions to properties, plants, equipment and mineral interests
Other investing activities, net	15,579
Dividends paid to stockholders
Borrowings on debt
Payments on debt
Other financing activity	21,364	29,818
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period
Cash, cash equivalents and restricted cash and cash equivalents at end of period
Parent Company [Member] | Reportable Legal Entities [Member]
Cash flows from operating activities	21,183	40,953
Additions to properties, plants, equipment and mineral interests
Other investing activities, net	(16,260)	(7,479)
Dividends paid to stockholders	(1,136)	(1,127)
Borrowings on debt	31,024
Payments on debt
Other financing activity	(1,186)	(41,096)
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents	33,625	(8,749)
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	103,878	113,275
Cash, cash equivalents and restricted cash and cash equivalents at end of period	137,503	104,526
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Cash flows from operating activities	18,747	11,508
Additions to properties, plants, equipment and mineral interests	(8,082)	(7,540)
Other investing activities, net	151	61
Dividends paid to stockholders
Borrowings on debt
Payments on debt	(644)	(1,658)
Other financing activity	(20,118)	3,025
Effect of exchange rate changes on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents	(9,946)	5,396
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	32,048	26,588
Cash, cash equivalents and restricted cash and cash equivalents at end of period	22,102	31,984
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Cash flows from operating activities	13,396	15,642
Additions to properties, plants, equipment and mineral interests	(9,553)	(14,118)
Other investing activities, net
Dividends paid to stockholders
Borrowings on debt
Payments on debt	(678)	(407)
Other financing activity	(1,285)	7,431
Effect of exchange rate changes on cash	876	1,814
Changes in cash, cash equivalents and restricted cash and cash equivalents	2,756	10,362
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	51,213	32,114
Cash, cash equivalents and restricted cash and cash equivalents at end of period	$ 53,969	$ 42,476