Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash and cash equivalents	$ 172,923	$ 169,777	$ 167,844	$ 155,209
Other current assets	138,412	121,314
Properties, plants, and equipment - net	2,025,607	2,032,685
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	59,241	47,901
Identifiable assets	2,396,183	2,371,677
Current liabilities	115,797	126,225
Long-term debt	509,353	506,817
Non-current portion of accrued reclamation	80,758	79,927
Non-current deferred tax liability	122,723	122,855
Other non-current liabilities	54,611	56,009
Shareholders' equity	1,512,941	1,479,844
Total liabilities and shareholders' equity	2,396,183	2,371,677
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	101,061	113,275	109,756	94,167
Other current assets	47,514	33,950
Properties, plants, and equipment - net	1,964	2,103
Intercompany receivable (payable)	461,542	404,121
Investments in subsidiaries	1,491,449	1,496,787
Other non-current assets	6,321	4,186
Identifiable assets	2,109,851	2,054,422
Current liabilities	46,720	22,401
Long-term debt	501,917	500,979
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	48,273	51,198
Shareholders' equity	1,512,941	1,479,844
Total liabilities and shareholders' equity	2,109,851	2,054,422
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	18,124	24,388	39,255	42,692
Other current assets	51,190	52,400
Properties, plants, and equipment - net	1,248,762	1,258,890
Intercompany receivable (payable)	(222,677)	(222,072)
Investments in subsidiaries
Other non-current assets	199,794	199,957
Identifiable assets	1,295,193	1,313,563
Current liabilities	56,189	86,730
Long-term debt	3,115	3,065
Non-current portion of accrued reclamation	61,964	63,025
Non-current deferred tax liability	13,349	14,212
Other non-current liabilities	5,363	5,108
Shareholders' equity	1,155,213	1,141,423
Total liabilities and shareholders' equity	1,295,193	1,313,563
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	53,738	32,114	18,833	18,350
Other current assets	40,283	35,537
Properties, plants, and equipment - net	774,881	771,692
Intercompany receivable (payable)	(351,019)	(307,018)
Investments in subsidiaries
Other non-current assets	6,906	5,337
Identifiable assets	524,789	537,662
Current liabilities	38,183	40,093
Long-term debt	4,321	2,773
Non-current portion of accrued reclamation	18,794	16,902
Non-current deferred tax liability	126,280	122,855
Other non-current liabilities	975	(325)
Shareholders' equity	336,236	355,364
Total liabilities and shareholders' equity	524,789	537,662
Consolidation, Eliminations [Member]
Cash and cash equivalents
Other current assets	(575)	(573)
Properties, plants, and equipment - net
Intercompany receivable (payable)	112,154	124,969
Investments in subsidiaries	(1,491,449)	(1,496,787)
Other non-current assets	(153,780)	(161,579)
Identifiable assets	(1,533,650)	(1,533,970)
Current liabilities	(25,295)	(22,999)
Long-term debt
Non-current portion of accrued reclamation
Non-current deferred tax liability	(16,906)	(14,212)
Other non-current liabilities		28
Shareholders' equity	(1,491,449)	(1,496,787)
Total liabilities and shareholders' equity	$ (1,533,650)	$ (1,533,970)