Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 9,434		$ 27,389
Other current assets	135,984		136,761
Properties, plants, equipment and mineral interests, net	2,485,869		2,520,004
Intercompany receivable (payable)	0		0
Investments in subsidiaries	0		0
Other non-current assets	39,426		19,790
Total assets	2,670,713		2,703,944
Liabilities and Stockholders' Equity
Current liabilities	117,853		136,194
Long-term debt	607,090		540,670
Accrued reclamation and closure costs	103,782		104,979
Non-current deferred tax liability	148,338		173,537
Other non-current liabilities	54,422		57,601
Stockholders' equity	1,639,228	$ 1,670,570	1,690,963	$ 1,478,695	$ 1,471,897	$ 1,461,277
Total liabilities and shareholders' equity	2,670,713		2,703,944
Reportable Legal Entities [Member] | Parent Company [Member]
Assets
Cash and cash equivalents	14		6,265
Other current assets	12,108		6,388
Properties, plants, equipment and mineral interests, net	1,913		1,913
Intercompany receivable (payable)	152,061		171,905
Investments in subsidiaries	1,506,079		1,577,564
Other non-current assets	286,460		276,641
Total assets	1,958,635		2,040,676
Liabilities and Stockholders' Equity
Current liabilities	(314,607)		(234,133)
Long-term debt	586,667		532,799
Accrued reclamation and closure costs	0		0
Non-current deferred tax liability	0		0
Other non-current liabilities	47,347		51,047
Stockholders' equity	1,639,228		1,690,963
Total liabilities and shareholders' equity	1,958,635		2,040,676
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	2,107		8,661
Other current assets	62,222		69,574
Properties, plants, equipment and mineral interests, net	1,785,072		1,795,994
Intercompany receivable (payable)	(364,163)		(222,815)
Investments in subsidiaries	0		0
Other non-current assets	22,798		9,030
Total assets	1,508,036		1,660,444
Liabilities and Stockholders' Equity
Current liabilities	122,102		118,863
Long-term debt	18,226		141,870
Accrued reclamation and closure costs	89,209		94,602
Non-current deferred tax liability	66,781		64,639
Other non-current liabilities	6,116		5,659
Stockholders' equity	1,205,602		1,234,811
Total liabilities and shareholders' equity	1,508,036		1,660,444
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	7,313		12,463
Other current assets	61,728		60,868
Properties, plants, equipment and mineral interests, net	698,884		722,097
Intercompany receivable (payable)	(199,609)		(171,834)
Investments in subsidiaries	0		0
Other non-current assets	(117,873)		(122,969)
Total assets	450,443		500,625
Liabilities and Stockholders' Equity
Current liabilities	44,741		45,922
Long-term debt	2,197		1,989
Accrued reclamation and closure costs	14,573		10,377
Non-current deferred tax liability	87,496		98,689
Other non-current liabilities	959		895
Stockholders' equity	300,477		342,753
Total liabilities and shareholders' equity	450,443		500,625
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents	0		0
Other current assets	(74)		(69)
Properties, plants, equipment and mineral interests, net	0		0
Intercompany receivable (payable)	411,711		222,744
Investments in subsidiaries	(1,506,079)		(1,577,564)
Other non-current assets	(151,959)		(142,912)
Total assets	(1,246,401)		(1,497,801)
Liabilities and Stockholders' Equity
Current liabilities	265,617
Long-term debt	0		(135,988)
Accrued reclamation and closure costs	0		0
Non-current deferred tax liability	(5,939)
Other non-current liabilities	0		0
Stockholders' equity	(1,506,079)		(1,577,564)
Total liabilities and shareholders' equity	$ (1,246,401)		(1,497,801)
Segment Reconciling Items [Member]
Liabilities and Stockholders' Equity
Current liabilities			205,542
Non-current deferred tax liability			$ 10,209