Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Cash and cash equivalents	$ 98,669			$ 62,452
Other current assets	136,487			116,672
Properties, plants, equipment and mineral interests, net	2,342,038			2,423,698
Intercompany receivable (payable)	0			0
Investments in subsidiaries	0			0
Other non-current assets	37,956			34,486
Total assets	2,615,150		$ 2,615,150	2,637,308
Current liabilities	125,391			116,974
Long-term debt	512,077			522,761
Accrued reclamation and closure costs	100,072			103,793
Non-current deferred tax liability	129,506			138,282
Other non-current liabilities	54,456			63,075
Shareholders' equity	1,693,648	$ 1,658,693		1,692,423	$ 1,616,644	$ 1,639,228	$ 1,690,963
Total liabilities and shareholders' equity	2,615,150			2,637,308
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	54,986			33,750
Other current assets	7,670			9,725
Properties, plants, equipment and mineral interests, net	1,913			1,913
Intercompany receivable (payable)	(46,498)			(28,381)
Investments in subsidiaries	1,665,459			1,636,802
Other non-current assets	286,403			289,422
Total assets	1,969,933			1,943,231
Current liabilities	268,234			309,293
Long-term debt	495,839			504,729
Accrued reclamation and closure costs	0			0
Non-current deferred tax liability	0			0
Other non-current liabilities	48,680			55,372
Shareholders' equity	1,693,648			1,692,423
Total liabilities and shareholders' equity	1,969,933			1,943,231
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	27,180			15,357
Other current assets	121,535			89,722
Properties, plants, equipment and mineral interests, net	2,331,450			2,410,458
Intercompany receivable (payable)	(508,931)			(579,830)
Investments in subsidiaries	0			0
Other non-current assets	25,194			24,325
Total assets	1,996,428			1,960,032
Current liabilities	(175,677)			(155,441)
Long-term debt	16,000			17,271
Accrued reclamation and closure costs	93,530			96,389
Non-current deferred tax liability	163,581			166,549
Other non-current liabilities	4,737			6,577
Shareholders' equity	1,542,903			1,517,805
Total liabilities and shareholders' equity	1,996,428			1,960,032
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	16,503			13,345
Other current assets	7,356			17,299
Properties, plants, equipment and mineral interests, net	8,675			11,327
Intercompany receivable (payable)	216,536			216,632
Investments in subsidiaries	0			0
Other non-current assets	(117,489)			(121,981)
Total assets	131,581			136,622
Current liabilities	(1,206)			(8,334)
Long-term debt	238			761
Accrued reclamation and closure costs	6,542			7,404
Non-current deferred tax liability	0			0
Other non-current liabilities	1,039			1,126
Shareholders' equity	122,556			118,997
Total liabilities and shareholders' equity	131,581			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
Intercompany receivable (payable)	338,893			391,579
Investments in subsidiaries	(1,665,459)			(1,636,802)
Other non-current assets	(156,152)			(157,280)
Total assets	(1,482,792)			(1,402,577)
Current liabilities	(216,742)			(262,492)
Long-term debt	0			0
Accrued reclamation and closure costs	0			0
Non-current deferred tax liability	(34,075)			(28,267)
Other non-current liabilities	0			0
Shareholders' equity	(1,665,459)			(1,636,802)
Total liabilities and shareholders' equity	$ (1,482,792)			$ (1,402,577)