Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income (loss)	$ (26,563)	$ (28,520)	$ 61,569
Non-cash elements included in net (loss) income:
Depreciation, depletion and amortization	140,905	126,467	124,918
Loss on disposition of investments		167
Unrealized loss on investments	2,816	251	177
Adjustment of inventory to market value	8,191		811
(Gain) loss on disposition of properties, plants, equipment and mineral interests	(2,793)	(6,042)	(147)
Provision for reclamation and closure costs	6,090	4,508	4,813
Deferred income taxes	(9,699)	19,392	2,774
Stock compensation	6,278	6,323	6,184
Acquisition costs			1,048
Amortization of loan origination fees	2,077	1,864	1,871
(Gain) loss on derivative contracts	(15,366)	20,741	(5,494)
Foreign exchange (gain) loss	(7,104)	10,208	4,460
Other non-cash items	(32)	51	(174)
Change in assets and liabilities, net of business acquired:
Accounts receivable	9,843	(2,414)	4,233
Inventories	(27,512)	(3,744)	(5,697)
Other current and non-current assets	(1,726)	(11,595)	14,422
Accounts payable and accrued liabilities	17,795	(16,434)	(6,539)
Accrued payroll and related benefits	(2,425)	2,092	17,705
Accrued taxes	645	(2,234)	263
Accrued reclamation and closure costs and other non-current liabilities	(7,199)	(5,203)	(1,869)
Net cash provided by operating activities	94,221	115,878	225,328
Investing activities:
Additions to properties, plants, equipment and mineral interests	(136,933)	(98,038)	(164,788)
Proceeds from disposition of properties, plants and equipment	2,411	374	348
Insurance proceeds received for damaged property	4,377	7,745
Purchases of investments	(31,971)	(56,613)	(48,943)
Maturities of investments	64,895	49,969	18,649
Purchase of other companies, net of cash and restricted cash acquired	(139,326)		(2,730)
Net cash used in investing activities	(236,547)	(96,563)	(197,464)
Financing activities:
Proceeds from issuance of common stock, net of offering costs	6,744	9,610	8,121
Dividends paid to common stockholders	(4,393)	(3,976)	(3,867)
Dividend paid to preferred stockholders	(552)	(552)	(552)
Debt issuance and credit facility fees paid	(2,638)	(476)	(127)
Acquisition of treasury shares	(2,694)	(2,868)	(4,440)
Borrowings under debt arrangements	102,024
Repayments of debt	(106,036)	(470)	(2,721)
Repayments of capital leases	(7,339)	(6,516)	(8,435)
Net cash used in financing activities	(14,884)	(5,248)	(12,021)
Effect of exchange rates on cash	(1,515)	1,095	(74)
Net (decrease) increase in cash, cash equivalents and restricted cash and cash equivalents	(158,725)	15,162	15,769
Cash, cash equivalents and restricted cash and cash equivalents at beginning of year	187,139	171,977	156,208
Cash, cash equivalents and restricted cash and cash equivalents at end of year	28,414	187,139	171,977
Supplemental disclosure of cash flow information:
Common stock and warrants issued for acquisition of other companies	252,544		48,109
Capital leases acquired	7,008	6,439	2,297
Common stock contributed to pension plans	7,595		7,687
Payment of accrued compensation in restricted stock units	4,863	4,240	5,511
Cash received (paid) during year for:
Interest, net of amount capitalized	(38,400)	(35,624)	(19,280)
Income tax (payments) receipts	$ 115	$ (23,128)	$ (6,333)