Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Sales of products	$ 673,266	$ 567,137	$ 577,775
Cost of sales and other direct production costs	450,349	353,994	304,727
Depreciation, depletion and amortization	199,518	134,044	120,599
Total cost of sales	649,867	488,038	425,326
Gross profit	23,399	79,099	152,449
Other operating expenses:
General and administrative	35,832	36,542	35,611
Exploration	15,919	35,695	23,510
Pre-development	3,150	4,887	5,448
Research and development	535	5,441	3,276
Provision for closed operations and environmental matters	4,690	6,119	6,701
Other operating expense	3,043	1,596	2,513
Loss (gain) on disposition of properties, plants, equipment and mineral interests	4,643	(2,793)	(6,042)
Suspension-related costs	12,051	20,693	21,301
Acquisition costs	645	10,045	25
Total other operating expense	80,508	118,225	92,343
(Loss) income from operations	(57,109)	(39,126)	60,106
Other income (expense):
(Loss) gain on derivative contracts	(3,971)	40,253	(21,250)
Gain (loss) on disposition of investments	923	(34)	(166)
Unrealized loss on investments	(2,389)	(2,816)	(247)
Net foreign exchange gain (loss)	(8,236)	10,310	(9,680)
Other net (expense) income	(4,429)	(907)	1,692
Interest expense, net of amount capitalized	(48,447)	(40,944)	(38,012)
Total other (expense) income:	(66,549)	5,862	(67,663)
Loss before income taxes	(123,658)	(33,264)	(7,557)
Income tax benefit (provision)	24,101	6,701	(20,963)
Net loss	(99,557)	(26,563)	(28,520)
Preferred stock dividends	(552)	(552)	(552)
Loss applicable to common stockholders	(100,109)	(27,115)	(29,072)
Comprehensive loss:
Net income (loss)	(99,557)	(26,563)	(28,520)
Unrealized loss and amortization of prior service on pension plans	(3,277)	(1,550)	(1,326)
Unrealized (loss) gain on derivative contracts designated as hedge transactions	8,436	(13,814)	10,290
Unrealized holding (losses) gains on investments	0	(2,443)	2,098
Reclassification of disposition or impairment of investments included in net loss	0	0	167
Other comprehensive income (loss)	5,159	(17,807)	11,229
Comprehensive loss	$ (94,398)	$ (44,370)	$ (17,291)
Basic loss per common share after preferred dividends (in dollars per share)	$ (0.20)	$ (0.06)	$ (0.07)
Diluted loss per common share after preferred dividends (in dollars per share)	$ (0.20)	$ (0.06)	$ (0.07)
Weighted average number of common shares outstanding – basic (in shares)	490,449	433,419	397,394
Weighted average number of common shares outstanding – diluted (in shares)	490,449	433,419	397,394