Note 15 - Guarantor Subsidiaries - Unaudited Interim Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities	$ 38,285	$ 45,821	$ 75,210	$ 74,115	$ 115,878	$ 225,328
Additions to properties, plants, and equipment			(83,285)	(70,390)
Acquisition of Klondex, net of cash and restricted cash acquired			(139,326)
Other investing activities, net			38,023	32
Dividends paid to shareholders			(3,607)	(3,392)
Borrowings on debt			78,024
Proceeds from (payments on) debt			(88,028)	(5,535)
Other financing activity, net			(2,069)	6,141
Effect of exchange rates on cash			(215)	1,051
Changes in cash, cash equivalents and restricted cash and cash equivalents			(125,273)	2,022
Beginning cash, cash equivalents and restricted cash and cash equivalents	171,977	171,977	186,107	171,977	171,977
Ending cash, cash equivalents and restricted cash and cash equivalents			60,856	173,999	186,107	171,977
Additions to properties, plants, equipment and mineral interests			(83,285)	(70,390)
Acquisitions of other companies, net of cash acquired			(139,326)
Proceeds from (payments on) debt			88,028	5,535
Reportable Legal Entities [Member] | Parent Company [Member]
Cash flows from operating activities			(52,473)	35,764
Additions to properties, plants, and equipment
Acquisition of Klondex, net of cash and restricted cash acquired			(139,326)
Other investing activities, net			(375,495)	176
Dividends paid to shareholders			(3,607)	(3,392)
Borrowings on debt			78,024
Proceeds from (payments on) debt			(47,000)
Other financing activity, net			464,080	(44,762)
Effect of exchange rates on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents			(75,797)	(12,214)
Beginning cash, cash equivalents and restricted cash and cash equivalents	113,275	113,275	103,878	113,275	113,275
Ending cash, cash equivalents and restricted cash and cash equivalents			28,081	101,061	103,878	113,275
Additions to properties, plants, equipment and mineral interests
Acquisitions of other companies, net of cash acquired			(139,326)
Proceeds from (payments on) debt			47,000
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Cash flows from operating activities			63,796	41,071
Additions to properties, plants, and equipment			(39,740)	(28,220)
Acquisition of Klondex, net of cash and restricted cash acquired
Other investing activities, net			4,603	5,779
Dividends paid to shareholders
Borrowings on debt
Proceeds from (payments on) debt			(3,094)	(4,518)
Other financing activity, net			(50,409)	(21,500)
Effect of exchange rates on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents			(24,844)	(7,388)
Beginning cash, cash equivalents and restricted cash and cash equivalents	26,588	26,588	32,048	26,588	26,588
Ending cash, cash equivalents and restricted cash and cash equivalents			7,204	19,200	32,048	26,588
Additions to properties, plants, equipment and mineral interests			(39,740)	(28,220)
Acquisitions of other companies, net of cash acquired
Proceeds from (payments on) debt			3,094	4,518
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities			(15,107)	21,435
Additions to properties, plants, and equipment			(43,545)	(42,170)
Acquisition of Klondex, net of cash and restricted cash acquired
Other investing activities, net			(294)	(584)
Dividends paid to shareholders
Borrowings on debt
Proceeds from (payments on) debt			(37,934)	(1,017)
Other financing activity, net			72,463	42,909
Effect of exchange rates on cash			(215)	1,051
Changes in cash, cash equivalents and restricted cash and cash equivalents			(24,632)	21,624
Beginning cash, cash equivalents and restricted cash and cash equivalents	32,114	32,114	51,213	32,114	32,114
Ending cash, cash equivalents and restricted cash and cash equivalents			26,581	53,738	51,213	32,114
Additions to properties, plants, equipment and mineral interests			(43,545)	(42,170)
Acquisitions of other companies, net of cash acquired
Proceeds from (payments on) debt			37,934	1,017
Consolidation, Eliminations [Member]
Cash flows from operating activities			78,994	(24,155)
Additions to properties, plants, and equipment
Acquisition of Klondex, net of cash and restricted cash acquired
Other investing activities, net			409,209	(5,339)
Dividends paid to shareholders
Borrowings on debt
Proceeds from (payments on) debt
Other financing activity, net			(488,203)	29,494
Effect of exchange rates on cash
Changes in cash, cash equivalents and restricted cash and cash equivalents
Beginning cash, cash equivalents and restricted cash and cash equivalents
Ending cash, cash equivalents and restricted cash and cash equivalents
Additions to properties, plants, equipment and mineral interests
Acquisitions of other companies, net of cash acquired
Proceeds from (payments on) debt