Note 13 - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Parent Company [Member]
Assets
Cash and cash equivalents	$ 91,039	$ 94,167	$ 145,284	$ 146,885
Other current assets	23,891	15,972
Properties, plants, equipment and mineral interests, net	2,178	2,281
Intercompany receivable (payable)	556,622	540,665
Investments in subsidiaries	1,249,577	1,252,191
Other non-current assets and deferred charges	2,112	2,200
Total assets	1,925,419	1,907,476
Liabilities and Stockholders' Equity
Current liabilities	33,512	21,087
Long-term debt	$ 500,042	$ 499,729
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	$ 44,619			$ 47,734
Stockholders' equity	1,347,246			1,338,926
Total liabilities and stockholders' equity	1,925,419			1,907,476
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	27,330	$ 42,692	35,957	33,824
Other current assets	87,718	58,453
Properties, plants, equipment and mineral interests, net	1,153,465	1,147,770
Intercompany receivable (payable)	$ (317,914)	$ (301,291)
Investments in subsidiaries
Other non-current assets and deferred charges	$ 189,146	$ 165,080
Total assets	1,139,745	1,112,704
Liabilities and Stockholders' Equity
Current liabilities	81,459	84,559
Long-term debt	6,613	6,128
Non-current portion of accrued reclamation	46,167	45,494
Non-current deferred tax liability	11,646			3,264
Other non-current liabilities	5,924			5,834
Stockholders' equity	987,936			967,425
Total liabilities and stockholders' equity	1,139,745			1,112,704
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	15,649	18,350	$ 14,990	28,956
Other current assets	49,855	32,273
Properties, plants, equipment and mineral interests, net	752,132	746,760
Intercompany receivable (payable)	$ (351,338)	$ (332,553)
Investments in subsidiaries
Other non-current assets and deferred charges	$ 2,971	$ 1,781
Total assets	469,269	466,611
Liabilities and Stockholders' Equity
Current liabilities	43,170	30,636
Long-term debt	1,303	3,183
Non-current portion of accrued reclamation	29,562	$ 29,055
Non-current deferred tax liability	134,723			119,836
Other non-current liabilities	(1,130)			(865)
Stockholders' equity	261,641			284,766
Total liabilities and stockholders' equity	$ 469,269			$ 466,611
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents
Other current assets	$ (18,628)	$ 7,626
Properties, plants, equipment and mineral interests, net
Intercompany receivable (payable)	$ 112,630	$ 93,179
Investments in subsidiaries	(1,249,577)	(1,252,191)
Other non-current assets and deferred charges	(139,685)	(113,480)
Total assets	(1,295,260)	(1,264,866)
Liabilities and Stockholders' Equity
Current liabilities	$ (25,323)	$ (9,198)
Long-term debt
Non-current portion of accrued reclamation
Non-current deferred tax liability	$ (20,360)			$ (3,477)
Other non-current liabilities
Stockholders' equity	$ (1,249,577)			$ (1,252,191)
Total liabilities and stockholders' equity	(1,295,260)			(1,264,866)
Consolidated [Member]
Assets
Cash and cash equivalents	134,018	$ 155,209	$ 196,231	209,665
Other current assets	142,836	114,324
Properties, plants, equipment and mineral interests, net	$ 1,907,775	$ 1,896,811
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets and deferred charges	$ 54,544	$ 55,581
Total assets	2,239,173	2,221,925
Liabilities and Stockholders' Equity
Current liabilities	132,818	127,084
Long-term debt	507,958	509,040
Non-current portion of accrued reclamation	75,729	74,549
Non-current deferred tax liability	126,009			119,623
Other non-current liabilities	49,413			52,703
Stockholders' equity	1,347,246			1,338,926
Total liabilities and stockholders' equity	2,239,173			2,221,925
Cash and cash equivalents	134,018	155,209	$ 196,231	$ 209,665
Other current assets	9,289	9,453
Properties, plants, equipment and mineral interests, net	1,907,775	1,896,811
Other non-current assets and deferred charges	2,783	2,783
Total assets	2,239,173	2,221,925
Current liabilities	132,818	127,084
Long-term debt	500,531	500,199
Non-current deferred tax liability	126,009	119,623
Other non-current liabilities	3,539	6,190
Stockholders' equity	1,347,246	1,338,926
Total liabilities and stockholders' equity	$ 2,239,173	$ 2,221,925