Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities	$ 115,892	$ 63,609
Additions to properties, plants, equipment and mineral interests	(54,382)	(97,338)
Other investing activities, net	(1,356)	1,457
Dividends paid to shareholders	(4,365)	(4,070)
Borrowings on debt	707,107	245,000
Payments on debt	(720,746)	(200,484)
Other financing activity, net	(4,032)	(2,825)
Effect of exchange rates on cash	(1,873)	257
Changes in cash, cash equivalents and restricted cash and cash equivalents	36,245	5,606
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	63,477	28,414
Cash, cash equivalents and restricted cash and cash equivalents at end of period	99,722	34,020
Acquisitions of other companies, net of cash acquired		0
Reportable Legal Entities [Member] | Parent Company [Member]
Cash flows from operating activities	47,945	(140,110)
Additions to properties, plants, equipment and mineral interests	0	0
Other investing activities, net	(28,657)	72,128
Dividends paid to shareholders	(4,365)	(4,070)
Borrowings on debt	707,107	245,000
Payments on debt	(716,500)	(195,000)
Other financing activity, net	15,706	34,769
Effect of exchange rates on cash	0	0
Changes in cash, cash equivalents and restricted cash and cash equivalents	21,236	12,717
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	33,750	6,266
Cash, cash equivalents and restricted cash and cash equivalents at end of period	54,986	18,983
Acquisitions of other companies, net of cash acquired		0
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash flows from operating activities	144,109	92,042
Additions to properties, plants, equipment and mineral interests	(53,846)	(56,329)
Other investing activities, net	262	42
Dividends paid to shareholders	0	0
Borrowings on debt	0	0
Payments on debt	(4,246)	(4,965)
Other financing activity, net	(73,632)	(38,672)
Effect of exchange rates on cash	(796)	257
Changes in cash, cash equivalents and restricted cash and cash equivalents	11,851	(7,625)
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	16,382	18,258
Cash, cash equivalents and restricted cash and cash equivalents at end of period	28,233	10,633
Acquisitions of other companies, net of cash acquired
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities	23,494	(15,242)
Additions to properties, plants, equipment and mineral interests	(536)	(41,009)
Other investing activities, net	(1,617)	1,415
Dividends paid to shareholders	0	0
Borrowings on debt	0	0
Payments on debt	0	(519)
Other financing activity, net	(17,106)	55,869
Effect of exchange rates on cash	(1,077)	0
Changes in cash, cash equivalents and restricted cash and cash equivalents	3,158	514
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	13,345	3,890
Cash, cash equivalents and restricted cash and cash equivalents at end of period	16,503	4,404
Acquisitions of other companies, net of cash acquired		0
Consolidation, Eliminations [Member]
Cash flows from operating activities	(99,656)	126,919
Additions to properties, plants, equipment and mineral interests	0	0
Other investing activities, net	28,656	(72,128)
Dividends paid to shareholders	0	0
Borrowings on debt	0	0
Payments on debt	0	0
Other financing activity, net	71,000	(54,791)
Effect of exchange rates on cash	0	0
Changes in cash, cash equivalents and restricted cash and cash equivalents	0	0
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	0	0
Cash, cash equivalents and restricted cash and cash equivalents at end of period	$ 0	0
Acquisitions of other companies, net of cash acquired		$ 0