Note 15 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 60,856	$ 186,107	$ 173,999	$ 171,977
Other current assets	137,473	135,129
Properties, plants, and equipment - net	2,487,429	1,999,311
Intercompany receivable (payable)
Investments in subsidiaries
Other non-current assets	24,500	24,611
Total assets	2,710,258	2,345,158
Current liabilities	132,456	112,130
Long-term debt	542,705	508,422
Non-current portion of accrued reclamation	99,314	79,366
Non-current deferred tax liability	164,928	124,352
Other non-current liabilities	48,786	59,611
Shareholders' equity	1,722,069	1,461,277
Total liabilities and shareholders' equity	2,710,258	2,345,158
Reportable Legal Entities [Member] | Parent Company [Member]
Cash and cash equivalents	28,081	103,878	101,061	113,275
Other current assets	15,219	47,555
Properties, plants, and equipment - net	1,920	1,946
Intercompany receivable (payable)	256,831	287,310
Investments in subsidiaries	1,767,234	1,358,025
Other non-current assets	149,034	14,409
Total assets	2,218,319	1,813,123
Current liabilities	(79,535)	(226,576)
Long-term debt	534,067	502,229
Non-current portion of accrued reclamation
Non-current deferred tax liability
Other non-current liabilities	41,718	53,588
Shareholders' equity	1,722,069	1,483,882
Total liabilities and shareholders' equity	2,218,319	1,813,123
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash and cash equivalents	7,204	32,048	19,200	26,588
Other current assets	42,515	47,608
Properties, plants, and equipment - net	1,246,274	1,244,161
Intercompany receivable (payable)	(222,897)	(177,438)
Investments in subsidiaries
Other non-current assets	4,390	7,289
Total assets	1,076,480	1,152,636
Current liabilities	77,328	66,550
Long-term debt	4,911	2,303
Non-current portion of accrued reclamation	67,145	67,565
Non-current deferred tax liability	16,497	10,120
Other non-current liabilities	5,784	5,185
Shareholders' equity	904,815	1,000,913
Total liabilities and shareholders' equity	1,076,480	1,152,636
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash and cash equivalents	26,581	51,213	53,738	32,114
Other current assets	79,807	40,541
Properties, plants, and equipment - net	1,239,235	753,204
Intercompany receivable (payable)	(138,477)	(341,182)
Investments in subsidiaries
Other non-current assets	11,670	9,283
Total assets	1,218,812	513,059
Current liabilities	55,389	37,671
Long-term debt	137,938	3,890
Non-current portion of accrued reclamation	32,169	11,801
Non-current deferred tax liability	159,613	124,352
Other non-current liabilities	1,284	838
Shareholders' equity	862,419	334,507
Total liabilities and shareholders' equity	1,248,812	513,059
Consolidation, Eliminations [Member]
Cash and cash equivalents
Other current assets	(68)	(575)
Properties, plants, and equipment - net
Intercompany receivable (payable)	104,543	231,310
Investments in subsidiaries	(1,767,234)	(1,358,025)
Other non-current assets	(140,594)	(6,370)
Total assets	(1,803,353)	(1,133,660)
Current liabilities	79,274	234,485
Long-term debt	(134,211)
Non-current portion of accrued reclamation
Non-current deferred tax liability	(11,182)	(10,120)
Other non-current liabilities
Shareholders' equity	(1,767,234)	(1,358,025)
Total liabilities and shareholders' equity	$ (1,833,353)	$ (1,133,660)