Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Properties, plants, and equipment - net	$ 2,033,506	$ 2,032,685
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	3,437	4,884
Identifiable assets	2,379,156	2,371,677
Current liabilities	113,788	126,225
Long-term debt	501,604	500,979
Non-current portion of accrued reclamation	79,280	79,927
Non-current deferred tax liability	121,260	122,855
Other non-current liabilities	6,414	11,518
Stockholders' equity	1,502,386	1,479,844
Total liabilities and shareholders' equity	2,379,156	2,371,677
Cash and cash equivalents	164,113	169,777	$ 143,613	$ 155,209
Other current assets	8,955	12,125
Reportable Legal Entities [Member] | Parent Company [Member]
Properties, plants, and equipment - net	1,986	2,103
Intercompany receivable (payable)	432,177	404,121
Investments in subsidiaries	1,496,848	1,496,787
Other non-current assets	2,606	4,186
Identifiable assets	2,086,825	2,054,422
Current liabilities	35,523	22,401
Long-term debt	501,604	500,979
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	47,312	51,198
Stockholders' equity	1,502,386	1,479,844
Total liabilities and shareholders' equity	2,086,825	2,054,422
Cash and cash equivalents	100,480	113,275	102,488	94,167
Other current assets	52,728	33,950
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Properties, plants, and equipment - net	1,256,606	1,258,890
Intercompany receivable (payable)	(229,725)	(222,072)
Investments in subsidiaries
Other non-current assets	193,833	199,957
Identifiable assets	1,284,161	1,313,563
Current liabilities	51,195	86,730
Long-term debt	3,838	3,065
Non-current portion of accrued reclamation	60,849	63,025
Non-current deferred tax liability	12,630	14,212
Other non-current liabilities	5,547	5,108
Stockholders' equity	1,150,102	1,141,423
Total liabilities and shareholders' equity	1,284,161	1,313,563
Cash and cash equivalents	26,770	24,387	23,720	42,692
Other current assets	36,677	52,400
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Properties, plants, and equipment - net	774,914	771,692
Intercompany receivable (payable)	(330,748)	(307,018)
Investments in subsidiaries
Other non-current assets	5,294	5,337
Identifiable assets	525,139	537,662
Current liabilities	32,735	40,093
Long-term debt	3,375	2,773
Non-current portion of accrued reclamation	18,431	16,902
Non-current deferred tax liability	123,086	122,855
Other non-current liabilities	766	(325)
Stockholders' equity	346,746	355,364
Total liabilities and shareholders' equity	525,139	537,662
Cash and cash equivalents	36,863	32,115	17,405	18,350
Other current assets	38,816	35,537
Consolidation, Eliminations [Member]
Properties, plants, and equipment - net
Intercompany receivable (payable)	128,296	124,969
Investments in subsidiaries	(1,496,848)	(1,496,787)
Other non-current assets	(147,841)	(161,579)
Identifiable assets	(1,516,969)	(1,533,970)
Current liabilities	(5,665)	(22,999)
Long-term debt
Non-current portion of accrued reclamation
Non-current deferred tax liability	(14,456)	(14,212)
Other non-current liabilities		28
Stockholders' equity	(1,496,848)	(1,496,787)
Total liabilities and shareholders' equity	(1,516,969)	(1,533,970)
Cash and cash equivalents
Other current assets	$ (576)	$ (573)