Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net (loss) income	$ (23,184)	$ 314	$ 25,082	$ 134	$ (2,870)	$ 447	$ (28,520)	$ 61,569
Non-cash elements included in net (loss) income:
Depreciation, depletion and amortization			31,073	59,855	108,814	91,255	126,467	124,918
Loss on disposition of investments						167
Unrealized loss on investments	2,207	124			2,461	73
Adjustment of inventory to market value					7,232
Gain on disposition of properties, plants, equipment, and mineral interests					(3,374)	(4,924)
Provision for reclamation and closure costs					3,957	3,379
Stock compensation					4,672	4,943
Deferred income taxes			(20,963)	(21,571)	(4,637)	(23,467)	19,392	2,774
Amortization of loan origination fees					1,471	1,415
Loss on derivative contracts					(15,208)	16,718
Foreign exchange (gain) loss			504	4,396	(2,032)	10,520	10,208	4,460
Other non-cash items, net					(37)	(1)
Change in assets and liabilities, net of business acquisitions:
Accounts receivable					(4,424)	4,903
Inventories					(18,954)	(9,611)
Other current and non-current assets					(5,569)	(2,685)
Accounts payable and accrued liabilities					12,308	(7,759)
Accrued payroll and related benefits					(4,207)	(913)
Accrued taxes					845	(4,469)
Accrued reclamation and closure costs and other non-current liabilities					(5,238)	(5,876)
Cash provided by operating activities			38,285	45,821	75,210	74,115	115,878	225,328
Investing activities:
Additions to properties, plants, equipment and mineral interests					(83,285)	(70,390)
Acquisition of Klondex, net of cash and restricted cash acquired					(139,326)
Proceeds from disposition of properties, plants, equipment and mineral interests					722	151
Insurance proceeds received for damaged property					4,377	5,628
Purchases of investments					(31,971)	(36,916)
Maturities of investments					64,895	31,169
Net cash used in investing activities					(184,588)	(70,358)
Financing activities:
Proceeds from sale of common stock, net of offering costs					3,085	9,610
Acquisition of treasury shares					(2,694)	(2,993)
Dividends paid to common shareholders					(3,193)	(2,978)
Dividends paid to preferred shareholders					(414)	(414)
Credit availability and debt issuance fees					(2,460)	(476)
Borrowings on debt					78,024
Repayments of debt					(82,036)	(470)
Repayments of capital leases					(5,992)	(5,065)
Net cash used in financing activities					(15,680)	(2,786)
Effect of exchange rates on cash					(215)	1,051
Net (decrease) increase in cash, cash equivalents and restricted cash and cash equivalents					(125,273)	2,022
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period			$ 171,977	$ 171,977	187,139	171,977	171,977
Cash, cash equivalents and restricted cash and cash equivalents at end of period	$ 61,866	$ 173,999			61,866	173,999	$ 187,139	$ 171,977
Significant non-cash investing and financing activities:
Addition of capital lease obligations					7,008	6,439
Common stock issued for the acquisition of other companies					252,544
Payment of accrued compensation in restricted stock units					$ 4,863	$ 4,240