Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash and cash equivalents	$ 215,715	$ 62,452
Other current assets	126,827	116,672
Properties, plants, equipment and mineral interests, net	2,393,187	2,423,698
Intercompany receivable (payable)	0	0
Investments in subsidiaries	0	0
Other non-current assets	28,395	34,486
Total assets	2,764,124	2,637,308
Current liabilities	117,183	116,974
Long-term debt	694,970	522,761
Accrued reclamation and closure costs	97,509	103,793
Non-current deferred tax liability	126,237	138,282
Other non-current liabilities	71,342	63,075
Stockholders' equity	1,656,883	1,692,423	$ 1,670,570	$ 1,690,963
Total liabilities and shareholders' equity	2,764,124	2,637,308
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	190,274	33,750
Other current assets	19,131	9,725
Properties, plants, equipment and mineral interests, net	1,913	1,913
Intercompany receivable (payable)	(15,844)	(28,381)
Investments in subsidiaries	1,648,133	1,636,802
Other non-current assets	263,344	289,422
Total assets	2,106,951	1,943,231
Current liabilities	(292,781)	(309,293)
Long-term debt	679,021	504,729
Accrued reclamation and closure costs	0	0
Non-current deferred tax liability	0	0
Other non-current liabilities	63,828	55,372
Stockholders' equity	1,656,883	1,692,423
Total liabilities and shareholders' equity	2,106,951	1,943,231
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	16,984	15,357
Other current assets	95,981	89,722
Properties, plants, equipment and mineral interests, net	2,380,598	2,410,458
Intercompany receivable (payable)	(565,090)	(579,830)
Investments in subsidiaries	0	0
Other non-current assets	24,970	24,325
Total assets	1,953,443	1,960,032
Current liabilities	154,776	155,441
Long-term debt	15,406	17,271
Accrued reclamation and closure costs	91,478	96,389
Non-current deferred tax liability	154,664	166,549
Other non-current liabilities	6,575	6,577
Stockholders' equity	1,530,544	1,517,805
Total liabilities and shareholders' equity	1,953,443	1,960,032
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	8,457	13,345
Other current assets	11,788	17,299
Properties, plants, equipment and mineral interests, net	10,676	11,327
Intercompany receivable (payable)	224,183	216,632
Investments in subsidiaries	0	0
Other non-current assets	(124,320)	(121,981)
Total assets	130,784	136,622
Current liabilities	5,682	8,334
Long-term debt	543	761
Accrued reclamation and closure costs	6,031	7,404
Non-current deferred tax liability	0	0
Other non-current liabilities	939	1,126
Stockholders' equity	117,589	118,997
Total liabilities and shareholders' equity	130,784	136,622
Consolidation, Eliminations [Member]
Cash and cash equivalents	0	0
Other current assets	(73)	(74)
Properties, plants, equipment and mineral interests, net	0	0
Intercompany receivable (payable)	356,751	391,579
Investments in subsidiaries	(1,648,133)	(1,636,802)
Other non-current assets	(135,599)	(157,280)
Total assets	(1,427,054)	(1,402,577)
Current liabilities	249,506	262,492
Long-term debt	0	0
Accrued reclamation and closure costs	0	0
Non-current deferred tax liability	(28,427)	(28,267)
Other non-current liabilities	0	0
Stockholders' equity	(1,648,133)	(1,636,802)
Total liabilities and shareholders' equity	$ (1,427,054)	$ (1,402,577)