Note 16 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 62,452	$ 27,389
Other current assets	116,672	136,761
Properties, plants, equipment and mineral interests, net	2,423,698	2,520,004
Intercompany receivable (payable)	0	0
Investments in subsidiaries	0	0
Other non-current assets	34,486	19,790
Total assets	2,637,308	2,703,944	$ 2,345,158
Current liabilities	116,974	136,194
Long-term debt	522,761	540,670
Accrued reclamation and closure costs	103,793	104,979
Non-current deferred tax liability	138,282	173,537
Other non-current liabilities	63,075	57,601
Stockholders' equity	1,692,423	1,690,963	$ 1,461,277	$ 1,462,240
Total liabilities and shareholders' equity	2,637,308	2,703,944
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	33,750	6,266
Other current assets	9,725	6,388
Properties, plants, equipment and mineral interests, net	1,913	1,913
Intercompany receivable (payable)	(28,381)	171,908
Investments in subsidiaries	1,636,802	1,577,869
Other non-current assets	289,422	276,641
Total assets	1,943,231	2,040,985
Current liabilities	(309,293)	(233,824)
Long-term debt	504,729	532,799
Accrued reclamation and closure costs	0	0
Non-current deferred tax liability	0	0
Other non-current liabilities	55,372	51,047
Stockholders' equity	1,692,423	1,690,963
Total liabilities and shareholders' equity	1,943,231	2,040,985
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	15,357	17,233
Other current assets	89,722	105,900
Properties, plants, equipment and mineral interests, net	2,410,458	2,503,467
Intercompany receivable (payable)	(579,830)	(546,374)
Investments in subsidiaries	0	0
Other non-current assets	24,325	10,906
Total assets	1,960,032	2,091,132
Current liabilities	155,441	157,640
Long-term debt	17,271	143,858
Accrued reclamation and closure costs	96,389	100,445
Non-current deferred tax liability	166,549	163,328
Other non-current liabilities	6,577	5,641
Stockholders' equity	1,517,805	1,520,220
Total liabilities and shareholders' equity	1,960,032	2,091,132
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	13,345	3,890
Other current assets	17,299	24,542
Properties, plants, equipment and mineral interests, net	11,327	14,624
Intercompany receivable (payable)	216,632	152,031
Investments in subsidiaries	0	0
Other non-current assets	(121,981)	(124,845)
Total assets	136,622	70,242
Current liabilities	8,334	7,145
Long-term debt	761	1
Accrued reclamation and closure costs	7,404	4,534
Non-current deferred tax liability	0	0
Other non-current liabilities	1,126	913
Stockholders' equity	118,997	57,649
Total liabilities and shareholders' equity	136,622	70,242
Consolidation, Eliminations [Member]
Cash and cash equivalents	0	0
Other current assets	(74)	(69)
Properties, plants, equipment and mineral interests, net	0	0
Intercompany receivable (payable)	391,579	222,435
Investments in subsidiaries	(1,636,802)	(1,577,869)
Other non-current assets	(157,280)	(142,912)
Total assets	(1,402,577)	(1,498,415)
Current liabilities	262,492	205,233
Long-term debt	0	(135,988)
Accrued reclamation and closure costs	0	0
Non-current deferred tax liability	(28,267)	10,209
Other non-current liabilities	0	0
Stockholders' equity	(1,636,802)	(1,577,869)
Total liabilities and shareholders' equity	$ (1,402,577)	$ (1,498,415)