Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 11,797	$ 27,389
Other current assets	132,310	136,761
Properties, plants, equipment and mineral interests, net	2,508,981	2,520,004
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	41,517	19,790
Total assets	2,694,605	2,703,944
Liabilities and Stockholders' Equity
Current liabilities	146,821	136,194
Long-term debt	556,989	540,670
Accrued reclamation and closure costs	104,186	104,979
Non-current deferred tax liability	159,425	173,537
Other non-current liabilities	56,614	57,601
Stockholders' equity	1,670,570	1,690,963	$ 1,471,897	$ 1,461,277
Total liabilities and shareholders' equity	2,694,605	2,703,944
Reportable Legal Entities [Member] | Parent Company [Member]
Assets
Cash and cash equivalents	3,401	6,265
Other current assets	6,359	6,388
Properties, plants, equipment and mineral interests, net	1,913	1,913
Intercompany receivable (payable)	162,575	171,905
Investments in subsidiaries	1,554,448	1,577,564
Other non-current assets	278,379	276,641
Total assets	2,007,075	2,040,676
Liabilities and Stockholders' Equity
Current liabilities	(247,045)	(234,133)
Long-term debt	533,723	532,799
Accrued reclamation and closure costs
Non-current deferred tax liability
Other non-current liabilities	49,827	51,047
Stockholders' equity	1,670,570	1,690,963
Total liabilities and shareholders' equity	2,007,075	2,040,676
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	3,269	8,661
Other current assets	61,293	69,574
Properties, plants, equipment and mineral interests, net	1,792,578	1,795,994
Intercompany receivable (payable)	(342,867)	(222,815)
Investments in subsidiaries
Other non-current assets	23,540	9,030
Total assets	1,537,813	1,660,444
Liabilities and Stockholders' Equity
Current liabilities	118,535	118,863
Long-term debt	19,682	141,870
Accrued reclamation and closure costs	90,083	94,602
Non-current deferred tax liability	71,173	64,639
Other non-current liabilities	5,871	5,659
Stockholders' equity	1,232,469	1,234,811
Total liabilities and shareholders' equity	1,537,813	1,660,444
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	5,127	12,463
Other current assets	64,732	60,868
Properties, plants, equipment and mineral interests, net	714,490	722,097
Intercompany receivable (payable)	(182,748)	(171,834)
Investments in subsidiaries
Other non-current assets	(115,696)	(122,969)
Total assets	485,905	500,625
Liabilities and Stockholders' Equity
Current liabilities	49,545	45,922
Long-term debt	3,584	1,989
Accrued reclamation and closure costs	14,103	10,377
Non-current deferred tax liability	95,778	98,689
Other non-current liabilities	916	895
Stockholders' equity	321,979	342,753
Total liabilities and shareholders' equity	485,905	500,625
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents
Other current assets	(74)	(69)
Properties, plants, equipment and mineral interests, net
Intercompany receivable (payable)	363,040	222,744
Investments in subsidiaries	(1,554,448)	(1,577,564)
Other non-current assets	(144,706)	(142,912)
Total assets	(1,336,188)	(1,497,801)
Liabilities and Stockholders' Equity
Current liabilities	225,786	205,542
Long-term debt		(135,988)
Accrued reclamation and closure costs
Non-current deferred tax liability	(7,526)	10,209
Other non-current liabilities
Stockholders' equity	(1,554,448)	(1,577,564)
Total liabilities and shareholders' equity	$ (1,336,188)	$ (1,497,801)