Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash and cash equivalents	$ 75,923		$ 62,452
Other current assets	138,958		116,672
Properties, plants, equipment and mineral interests, net	2,354,883		2,423,698
Intercompany receivable (payable)	0		0
Investments in subsidiaries	0		0
Other non-current assets	33,644		34,486
Total assets	2,603,408		2,637,308
Current liabilities	115,636		116,974
Long-term debt	534,388		522,761
Accrued reclamation and closure costs	99,449		103,793
Non-current deferred tax liability	128,677		138,282
Other non-current liabilities	66,565		63,075
Stockholders' equity	1,658,693	$ 1,656,883	1,692,423	$ 1,639,228	$ 1,670,570	$ 1,690,963
Total liabilities and shareholders' equity	2,603,408		2,637,308
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	42,071		33,750
Other current assets	12,745		9,725
Properties, plants, equipment and mineral interests, net	1,913		1,913
Intercompany receivable (payable)	(25,989)		(28,381)
Investments in subsidiaries	1,643,530		1,636,802
Other non-current assets	277,344		289,422
Total assets	1,951,614		1,943,231
Current liabilities	(284,257)		(309,293)
Long-term debt	518,252		504,729
Accrued reclamation and closure costs	0		0
Non-current deferred tax liability	0		0
Other non-current liabilities	58,926		55,372
Stockholders' equity	1,658,693		1,692,423
Total liabilities and shareholders' equity	1,951,614		1,943,231
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	24,523		15,357
Other current assets	115,587		89,722
Properties, plants, equipment and mineral interests, net	2,343,623		2,410,458
Intercompany receivable (payable)	(559,649)		(579,830)
Investments in subsidiaries	0		0
Other non-current assets	24,666		24,325
Total assets	1,948,750		1,960,032
Current liabilities	150,204		155,441
Long-term debt	15,848		17,271
Accrued reclamation and closure costs	93,160		96,389
Non-current deferred tax liability	158,628		166,549
Other non-current liabilities	6,673		6,577
Stockholders' equity	1,524,237		1,517,805
Total liabilities and shareholders' equity	1,948,750		1,960,032
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	9,329		13,345
Other current assets	10,700		17,299
Properties, plants, equipment and mineral interests, net	9,347		11,327
Intercompany receivable (payable)	221,658		216,632
Investments in subsidiaries	0		0
Other non-current assets	(120,211)		(121,981)
Total assets	130,823		136,622
Current liabilities	3,987		8,334
Long-term debt	288		761
Accrued reclamation and closure costs	6,289		7,404
Non-current deferred tax liability	0		0
Other non-current liabilities	966		1,126
Stockholders' equity	119,293		118,997
Total liabilities and shareholders' equity	130,823		136,622
Consolidation, Eliminations [Member]
Cash and cash equivalents	0		0
Other current assets	(74)		(74)
Properties, plants, equipment and mineral interests, net	0		0
Investments in subsidiaries	(1,643,530)		(1,636,802)
Other non-current assets	(148,155)		(157,280)
Total assets	(1,427,779)		(1,402,577)
Long-term debt	0		0
Accrued reclamation and closure costs	0		0
Non-current deferred tax liability	(29,951)		(28,267)
Other non-current liabilities	0		0
Stockholders' equity	(1,643,530)		(1,636,802)
Total liabilities and shareholders' equity	(1,427,779)		(1,402,577)
Segment Reconciling Items [Member]
Intercompany receivable (payable)	363,980		391,579
Current liabilities	$ 245,702		$ 262,492