Note 14 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Identifiable assets:
Cash and cash equivalents	$ 212,569	$ 186,107
Other current assets	153,536	134,218
Properties, plants, and equipment - net	2,008,704	2,020,021
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	23,282	24,611
Identifiable assets	2,398,091	2,364,957
Liabilities and Stockholders' Equity
Current liabilities	115,934	112,130
Long-term debt	540,660	508,422
Accrued reclamation and closure costs	78,887	79,366
Non-current deferred tax liability	116,866	121,546
Other non-current liabilities	51,243	59,611
Stockholders' equity	1,494,501	1,483,882
Total liabilities and shareholders' equity	2,398,091	2,364,957
Reportable Legal Entities [Member] | Parent Company [Member]
Identifiable assets:
Cash and cash equivalents	137,504	103,878
Other current assets	48,067	47,555
Properties, plants, and equipment - net	1,934	1,946
Intercompany receivable (payable)	293,972	287,310
Investments in subsidiaries	1,373,604	1,358,025
Other non-current assets	13,807	14,409
Identifiable assets	1,868,888	1,813,123
Liabilities and Stockholders' Equity
Current liabilities	(203,947)	(226,576)
Long-term debt	533,566	502,229
Accrued reclamation and closure costs
Non-current deferred tax liability
Other non-current liabilities	44,768	53,588
Stockholders' equity	1,494,501	1,483,882
Total liabilities and shareholders' equity	1,868,888	1,813,123
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Identifiable assets:
Cash and cash equivalents	21,096	31,016
Other current assets	55,771	47,608
Properties, plants, and equipment - net	1,241,325	1,244,161
Intercompany receivable (payable)	(163,503)	(177,438)
Investments in subsidiaries
Other non-current assets	7,370	7,289
Identifiable assets	1,162,059	1,152,636
Liabilities and Stockholders' Equity
Current liabilities	67,735	66,550
Long-term debt	3,682	2,303
Accrued reclamation and closure costs	66,614	67,565
Non-current deferred tax liability	11,630	10,120
Other non-current liabilities	5,384	5,185
Stockholders' equity	1,007,014	1,000,913
Total liabilities and shareholders' equity	1,162,059	1,152,636
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Identifiable assets:
Cash and cash equivalents	53,969	51,213
Other current assets	49,767	39,630
Properties, plants, and equipment - net	765,445	773,914
Intercompany receivable (payable)	(338,811)	(341,182)
Investments in subsidiaries
Other non-current assets	8,325	9,283
Identifiable assets	538,695	532,858
Liabilities and Stockholders' Equity
Current liabilities	38,776	37,671
Long-term debt	3,412	3,890
Accrued reclamation and closure costs	12,273	11,801
Non-current deferred tax liability	116,553	121,546
Other non-current liabilities	1,091	838
Stockholders' equity	366,590	357,112
Total liabilities and shareholders' equity	538,695	532,858
Consolidation, Eliminations [Member]
Identifiable assets:
Cash and cash equivalents
Other current assets	(69)	(575)
Properties, plants, and equipment - net
Intercompany receivable (payable)	208,342	231,310
Investments in subsidiaries	(1,373,604)	(1,358,025)
Other non-current assets	(6,220)	(6,370)
Identifiable assets	(1,171,551)	(1,133,660)
Liabilities and Stockholders' Equity
Current liabilities	213,370	234,485
Long-term debt
Accrued reclamation and closure costs
Non-current deferred tax liability	(11,317)	(10,120)
Other non-current liabilities
Stockholders' equity	(1,373,604)	(1,358,025)
Total liabilities and shareholders' equity	$ (1,171,551)	$ (1,133,660)