Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Identifiable assets:
Cash and cash equivalents	$ 239,722	$ 186,107	$ 165,211	$ 171,977
Other current assets	112,640	134,218
Properties, plants, and equipment - net	2,006,592	2,020,021
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	33,640	24,611
Total assets	2,392,594	2,364,957
Liabilities and Stockholders' Equity
Current liabilities	105,333	112,130
Long-term debt	541,987	508,422
Non-current portion of accrued reclamation	78,102	79,366
Non-current deferred tax liability	112,462	121,546
Other non-current liabilities	53,411	59,611
Stockholders' equity	1,501,299	1,483,882
Total liabilities and shareholders' equity	2,392,594	2,364,957
Reportable Legal Entities [Member] | Parent Company [Member]
Identifiable assets:
Cash and cash equivalents	163,357	103,878	100,480	113,275
Other current assets	18,609	47,555
Properties, plants, and equipment - net	1,927	1,946
Intercompany receivable (payable)	281,519	287,310
Investments in subsidiaries	1,379,384	1,358,025
Other non-current assets	23,669	14,409
Total assets	1,868,465	1,813,123
Liabilities and Stockholders' Equity
Current liabilities	(212,784)	(226,576)
Long-term debt	533,230	502,229
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	46,720	53,588
Stockholders' equity	1,501,299	1,483,882
Total liabilities and shareholders' equity	1,868,465	1,813,123
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Identifiable assets:
Cash and cash equivalents	24,387	31,016	27,869	26,588
Other current assets	44,279	47,608
Properties, plants, and equipment - net	1,246,318	1,244,161
Intercompany receivable (payable)	(145,439)	(177,438)
Investments in subsidiaries
Other non-current assets	8,357	7,289
Total assets	1,177,902	1,152,636
Liabilities and Stockholders' Equity
Current liabilities	77,213	66,550
Long-term debt	5,825	2,303
Non-current portion of accrued reclamation	66,157	67,565
Non-current deferred tax liability	11,916	10,120
Other non-current liabilities	5,584	5,185
Stockholders' equity	1,011,207	1,000,913
Total liabilities and shareholders' equity	1,177,902	1,152,636
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Identifiable assets:
Cash and cash equivalents	51,978	51,213	36,862	32,114
Other current assets	49,822	39,630
Properties, plants, and equipment - net	758,347	773,914
Intercompany receivable (payable)	(332,954)	(341,182)
Investments in subsidiaries
Other non-current assets	8,296	9,283
Total assets	535,489	532,858
Liabilities and Stockholders' Equity
Current liabilities	39,180	37,671
Long-term debt	2,932	3,890
Non-current portion of accrued reclamation	11,945	11,801
Non-current deferred tax liability	112,148	121,546
Other non-current liabilities	1,107	838
Stockholders' equity	368,177	357,112
Total liabilities and shareholders' equity	535,489	532,858
Consolidation, Eliminations [Member]
Identifiable assets:
Cash and cash equivalents
Other current assets	(70)	(575)
Properties, plants, and equipment - net
Intercompany receivable (payable)	196,874	231,310
Investments in subsidiaries	(1,379,384)	(1,358,025)
Other non-current assets	(6,682)	(6,370)
Total assets	(1,189,262)	(1,133,660)
Liabilities and Stockholders' Equity
Current liabilities	201,724	234,485
Long-term debt
Non-current portion of accrued reclamation
Non-current deferred tax liability	(11,602)	(10,120)
Other non-current liabilities
Stockholders' equity	(1,379,384)	(1,358,025)
Total liabilities and shareholders' equity	$ (1,189,262)	$ (1,133,660)