Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities	$ 4,927	$ 20,030
Additions to properties, plants, equipment and mineral interests	(19,870)	(33,071)
Other investing activities, net	154	1
Dividends paid to stockholders	(1,442)	(1,347)
Borrowings on debt	679,500	58,000
Payments on debt	(507,784)	(59,261)
Other financing activity	(458)	(39)
Effect of exchange rate changes on cash	(1,736)	95
Changes in cash, cash equivalents and restricted cash and cash equivalents	153,291	(15,592)
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	216,768	12,822
Reportable Legal Entities [Member] | Parent Company [Member]
Cash flows from operating activities	(13,285)	(37,529)
Additions to properties, plants, equipment and mineral interests	0	0
Other investing activities, net	(11,331)	23,115
Dividends paid to stockholders	(1,442)	(1,347)
Borrowings on debt	679,500	58,000
Payments on debt	(506,500)	(58,000)
Other financing activity	9,582	12,896
Effect of exchange rate changes on cash	0	0
Changes in cash, cash equivalents and restricted cash and cash equivalents	156,524	(2,865)
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	190,274	3,401
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash flows from operating activities	33,035	29,554
Additions to properties, plants, equipment and mineral interests	(19,068)	(27,860)
Other investing activities, net	154	1
Dividends paid to stockholders	0	0
Borrowings on debt	0	0
Payments on debt	(1,284)	(1,261)
Other financing activity	(10,233)	(12,467)
Effect of exchange rate changes on cash	(949)	95
Changes in cash, cash equivalents and restricted cash and cash equivalents	1,655	(11,938)
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	18,037	6,320
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities	19,830	(11,315)
Additions to properties, plants, equipment and mineral interests	(802)	(5,211)
Other investing activities, net	0	0
Dividends paid to stockholders	0	0
Borrowings on debt	0	0
Payments on debt	0	0
Other financing activity	(23,129)	15,737
Effect of exchange rate changes on cash	(787)	0
Changes in cash, cash equivalents and restricted cash and cash equivalents	(4,888)	(789)
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	8,457	3,101
Consolidation, Eliminations [Member]
Cash flows from operating activities	(34,653)	39,320
Additions to properties, plants, equipment and mineral interests	0	0
Other investing activities, net	11,331	(23,115)
Dividends paid to stockholders	0	0
Borrowings on debt	0	0
Payments on debt	0	0
Other financing activity	23,322	(16,205)
Effect of exchange rate changes 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