Note 13 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash and cash equivalents	$ 176,786	$ 169,777	$ 134,018	$ 155,209
Other current assets	8,256	12,125
Properties, plants, and equipment - net	2,032,983	2,032,685
Other non-current assets	2,609	4,884
Total assets	2,406,461	2,371,677
Current liabilities	132,655	126,225
Long-term debt	501,292	500,979
Non-current deferred tax liability	121,025	122,855
Other non-current liabilities	5,321	11,518
Stockholders' equity	1,514,303	1,479,844
Total liabilities and stockholders' equity	2,406,461	2,371,677
Consolidation, Eliminations [Member]
Cash and cash equivalents
Other current assets	(574)	(1,236)
Properties, plants, and equipment - net
Intercompany receivable (payable)	105,077	124,969
Investments in subsidiaries	(1,499,401)	(1,496,787)
Other non-current assets	(137,787)	(160,916)
Total assets	(1,532,685)	(1,533,970)
Current liabilities	(15,906)	(22,999)
Long-term debt
Non-current portion of accrued reclamation
Non-current deferred tax liability	(17,377)	(14,212)
Other non-current liabilities		28
Stockholders' equity	(1,499,402)	(1,496,787)
Total liabilities and stockholders' equity	(1,532,685)	(1,533,970)
Parent Company [Member] | Reportable Legal Entities [Member]
Cash and cash equivalents	104,526	113,275	91,039	94,167
Other current assets	49,158	33,950
Properties, plants, and equipment - net	2,031	2,103
Intercompany receivable (payable)	453,210	404,121
Investments in subsidiaries	1,499,401	1,496,787
Other non-current assets	1,761	4,186
Total assets	2,110,087	2,054,422
Current liabilities	47,831	22,401
Long-term debt	501,292	500,979
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	46,661	51,198
Stockholders' equity	1,514,303	1,479,844
Total liabilities and stockholders' equity	2,110,087	2,054,422
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Cash and cash equivalents	29,783	24,388	27,330	42,692
Other current assets	51,092	65,369
Properties, plants, and equipment - net	1,259,079	1,258,890
Intercompany receivable (payable)	(243,979)	(222,072)
Investments in subsidiaries
Other non-current assets	188,815	186,988
Total assets	1,284,790	1,313,563
Current liabilities	52,762	86,730
Long-term debt	3,692	3,065
Non-current portion of accrued reclamation	61,706	63,025
Non-current deferred tax liability	11,767	14,212
Other non-current liabilities	5,328	5,108
Stockholders' equity	1,149,535	1,141,423
Total liabilities and stockholders' equity	1,284,790	1,313,563
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Cash and cash equivalents	42,477	32,114	$ 15,649	$ 18,350
Other current assets	38,693	35,524
Properties, plants, and equipment - net	771,873	771,692
Intercompany receivable (payable)	(314,308)	(307,018)
Investments in subsidiaries
Other non-current assets	5,534	5,350
Total assets	544,269	537,662
Current liabilities	47,968	41,348
Long-term debt	2,396	2,773
Non-current portion of accrued reclamation	17,628	16,902
Non-current deferred tax liability	126,635	121,600
Other non-current liabilities	(225)	(325)
Stockholders' equity	349,867	355,364
Total liabilities and stockholders' equity	544,269	537,662
Consolidated Entities [Member]
Cash and cash equivalents	176,786	169,777
Other current assets	138,369	133,607
Properties, plants, and equipment - net	2,032,983	2,032,685
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	58,323	35,608
Total assets	2,406,461	2,371,677
Current liabilities	132,655	127,480
Long-term debt	507,380	506,817
Non-current portion of accrued reclamation	79,334	79,927
Non-current deferred tax liability	121,025	121,600
Other non-current liabilities	51,764	56,009
Stockholders' equity	1,514,303	1,479,844
Total liabilities and stockholders' equity	$ 2,406,461	$ 2,371,677