Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Sales of products	$ 691,873	$ 673,266	$ 567,137
Cost of sales and other direct production costs	389,040	450,349	353,994
Depreciation, depletion and amortization	157,130	199,518	134,044
Total cost of sales	546,170	649,867	488,038
Gross profit	145,703	23,399	79,099
Other operating expenses:
General and administrative	35,561	35,832	36,542
Exploration	15,853	15,919	35,695
Pre-development	2,442	3,150	4,887
Research and development	0	535	5,441
Provision for closed operations and environmental matters	3,929	4,690	6,119
Other operating expense	8,864	3,043	1,596
Loss (gain) on disposition of properties, plants, equipment and mineral interests	572	4,643	(2,793)
Ramp-up and suspension costs	24,911	12,051	20,693
Foundation grant	1,970	0	0
Acquisition costs	20	645	10,045
Total other operating expense	94,122	80,508	118,225
Income (loss) from operations	51,581	(57,109)	(39,126)
Other income (expense):
(Loss) gain on derivative contracts	(22,074)	(3,971)	40,253
Gain (loss) on disposition of investments	0	923	(34)
Unrealized gain (loss) on investments	10,268	(2,389)	(2,816)
Net foreign exchange (loss) gain	(4,605)	(8,236)	10,310
Other net expense	(2,256)	(4,429)	(907)
Interest expense, net	(49,569)	(48,447)	(40,944)
Total other (expense) income:	(68,236)	(66,549)	5,862
Loss before income taxes	(16,655)	(123,658)	(33,264)
Income tax (provision) benefit	(135)	24,101	6,701
Net loss	(16,790)	(99,557)	(26,563)
Preferred stock dividends	(552)	(552)	(552)
Loss applicable to common stockholders	(17,342)	(100,109)	(27,115)
Comprehensive loss:
Net income (loss)	(16,790)	(99,557)	(26,563)
Unrealized loss and amortization of prior service on pension plans	(3,559)	(3,277)	(1,550)
Unrealized gain (loss) on derivative contracts designated as hedge transactions	7,980	8,436	(13,814)
Unrealized holding (losses) gains on investments	0	0	(2,443)
Total change in accumulated other comprehensive loss, net	4,421	5,159	(17,807)
Comprehensive loss	$ (12,369)	$ (94,398)	$ (44,370)
Basic loss per common share after preferred dividends (in dollars per share)	$ (0.03)	$ (0.20)	$ (0.06)
Diluted loss per common share after preferred dividends (in dollars per share)	$ (0.03)	$ (0.20)	$ (0.06)
Weighted average number of common shares outstanding – basic (in shares)	527,329	490,449	433,419
Weighted average number of common shares outstanding – diluted (in shares)	527,329	490,449	433,419