Note 3 - Properties, Plants, Equipment and Mineral Interests, and Lease Commitments - Major Components of Property, Plants, Equipment and Mineral Interests (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment, gross	$ 2,974,092	$ 2,874,778
Less accumulated depreciation, depletion and amortization	954,071	842,093
Net carrying value	2,020,021	2,032,685
Mining Properties Including Asset Retirement Obligations [Member]
Property, plant and equipment, gross	443,182	403,935
Mine Development [Member]
Property, plant and equipment, gross	390,094	347,356
Plant and Equipment [Member]
Property, plant and equipment, gross	847,462	814,091
Land [Member]
Property, plant and equipment, gross	26,949	26,229
Mineral Interests [Member]
Property, plant and equipment, gross	919,082	952,382
Construction in Progress [Member]
Property, plant and equipment, gross	$ 347,323	$ 330,785