Note 16 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 186,107	$ 169,777	$ 155,209	$ 209,665
Other current assets	134,218	121,314
Properties, plants, and equipment - net	2,020,021	2,032,685
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	24,611	47,901
Identifiable assets	2,364,957	2,371,677	2,221,925
Liabilities and Stockholders' Equity
Current liabilities	112,130	126,225
Long-term debt	508,422	506,817
Reclamation and closure costs, long-term	79,366	79,927
Deferred tax liability	121,546	122,855
Other non-current liabilities	59,611	56,009
Stockholders' equity	1,483,882	1,479,844	1,338,926	1,396,974
Total liabilities and shareholders' equity	2,364,957	2,371,677
Reportable Legal Entities [Member] | Parent Company [Member]
Assets
Cash and cash equivalents	103,878	113,275	94,167	146,885
Other current assets	47,555	33,950
Properties, plants, and equipment - net	1,946	2,103
Intercompany receivable (payable)	287,310	404,121
Investments in subsidiaries	1,358,025	1,496,787
Other non-current assets	14,409	4,186
Identifiable assets	1,813,123	2,054,422
Liabilities and Stockholders' Equity
Current liabilities	(226,576)	22,401
Long-term debt	502,229	500,979
Reclamation and closure costs, long-term
Deferred tax liability
Other non-current liabilities	53,588	51,198
Stockholders' equity	1,483,882	1,479,844
Total liabilities and shareholders' equity	1,813,123	2,054,422
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	31,016	24,388	42,692	33,824
Other current assets	47,608	52,400
Properties, plants, and equipment - net	1,244,161	1,258,890
Intercompany receivable (payable)	(177,438)	(222,072)
Investments in subsidiaries
Other non-current assets	7,289	199,957
Identifiable assets	1,152,636	1,313,563
Liabilities and Stockholders' Equity
Current liabilities	66,550	86,730
Long-term debt	2,303	3,065
Reclamation and closure costs, long-term	67,565	63,025
Deferred tax liability	10,120	14,212
Other non-current liabilities	5,185	5,108
Stockholders' equity	1,000,913	1,141,423
Total liabilities and shareholders' equity	1,152,636	1,313,563
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	51,213	32,114	18,350	28,956
Other current assets	39,630	35,537
Properties, plants, and equipment - net	773,914	771,692
Intercompany receivable (payable)	(341,182)	(307,018)
Investments in subsidiaries
Other non-current assets	9,283	5,337
Identifiable assets	532,858	537,662
Liabilities and Stockholders' Equity
Current liabilities	37,671	40,093
Long-term debt	3,890	2,773
Reclamation and closure costs, long-term	11,801	16,902
Deferred tax liability	121,546	122,855
Other non-current liabilities	838	(325)
Stockholders' equity	357,112	355,364
Total liabilities and shareholders' equity	532,858	537,662
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents
Other current assets	(575)	(573)
Properties, plants, and equipment - net
Intercompany receivable (payable)	231,310	124,969
Investments in subsidiaries	(1,358,025)	(1,496,787)
Other non-current assets	(6,370)	(161,579)
Identifiable assets	(1,133,660)	(1,533,970)
Liabilities and Stockholders' Equity
Current liabilities	234,485	(22,999)
Long-term debt
Reclamation and closure costs, long-term
Deferred tax liability	(10,120)	(14,212)
Other non-current liabilities		28
Stockholders' equity	(1,358,025)	(1,496,787)
Total liabilities and shareholders' equity	$ (1,133,660)	$ (1,533,970)