Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net Income (Loss)	$ (23,519)	$ 69,547	$ (86,968)
Non-cash elements included in net (loss) income:
Depreciation, depletion and amortization	121,930	117,413	112,585
Loss on disposition of investments	167		44
Unrealized loss on investments	251	177	3,333
Adjustment of inventory to market value		811	1,649
(Gain) loss on disposition of properties, plants, equipment and mineral interests	(6,042)	(147)	404
Provision for reclamation and closure costs	4,508	4,813	12,036
Deferred income taxes	18,308	2,112	54,978
Stock compensation	6,323	6,184	5,425
Acquisition costs		1,048
Amortization of loan origination fees	1,864	1,871	1,821
Loss (gains) on derivative contracts	20,741	(5,494)	11,630
Foreign exchange loss (gain)	10,828	4,649	(20,081)
Other non-cash items	51	(174)	(35)
Change in assets and liabilities, net of business acquired:
Accounts receivable	(2,414)	4,233	(6,834)
Inventories	(3,744)	(5,697)	(854)
Other current and non-current assets	(11,595)	14,422	1,195
Accounts payable and accrued liabilities	(16,434)	(6,539)	(4,211)
Accrued payroll and related benefits	2,092	17,705	7,325
Accrued taxes	(2,234)	263	4,653
Accrued reclamation and closure costs and other non-current liabilities	(5,203)	(1,869)	8,350
Net cash provided by operating activities	115,878	225,328	106,445
Investing activities:
Additions to properties, plants, equipment and mineral interests	(98,038)	(164,788)	(137,443)
Proceeds from sale of investments			14
Proceeds from disposition of properties, plants and equipment	374	348	579
Insurance proceeds received for damaged property	7,745
Redemptions of restricted cash and investment balances	1,168
Purchases of investments	(56,613)	(48,943)	(947)
Maturities of investments	49,969	18,649
Purchase of other companies, net of cash acquired		(3,931)	(809)
Net cash used by investing activities	(95,395)	(198,665)	(138,606)
Financing activities:
Proceeds from issuance of common stock, net of offering costs	9,610	8,121
Dividends paid to common stockholders	(3,976)	(3,867)	(3,739)
Dividend paid to preferred stockholders	(552)	(552)	(552)
Debt issuance and credit facility fees paid	(476)	(127)	(127)
Acquisition of treasury shares	(2,868)	(4,440)	(1,874)
Payments on debt	(470)	(2,721)	(870)
Payments on capital leases	(6,516)	(8,435)	(9,981)
Net cash (used in) provided by financing activities	(5,248)	(12,021)	(17,143)
Effect of exchange rates on cash	1,095	(74)	(5,152)
Net increase (decrease) in cash and cash equivalents	16,330	14,568	(54,456)
Cash and cash equivalents at beginning of year	169,777	155,209	209,665
Cash and cash equivalents at end of year	186,107	169,777	155,209
Supplemental disclosure of cash flow information:
Common stock issued for acquisition of other companies		48,109	19,133
Capital leases acquired	6,439	2,297	5,090
Payment of accrued compensation in restricted stock units	4,240	5,511	3,016
Cash received (paid) during year for:
Interest, net of amount capitalized	(35,624)	(19,280)	(22,628)
Income tax (payments) receipts	$ (23,128)	$ (6,333)	$ 4,186