Condensed Consolidated Statements of Operations and Comprehensive (Loss) Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Sales of products	$ 166,355	$ 134,172	$ 303,280	$ 286,789
Cost of sales and other direct production costs	92,853	104,938	178,740	215,324
Depreciation, depletion and amortization	39,423	49,477	79,089	88,264
Total cost of sales	132,276	154,415	257,829	303,588
Gross profit (loss)	34,079	(20,243)	45,451	(16,799)
Other operating expenses:
General and administrative	6,979	8,918	15,918	18,877
Exploration	1,962	4,346	4,492	8,748
Pre-development	563	798	1,098	1,654
Research and development	0	158	0	561
Other operating expense	1,439	657	2,354	1,244
Loss on disposition or impairment of properties, plants, equipment and mineral interests	677	4,642	573	4,642
Provision for closed operations and environmental matters	1,037	1,052	1,553	1,622
Ramp-up and suspension costs	9,572	2,266	22,568	5,044
Foundation grant	1,970	0	1,970	0
Acquisition costs	6	397	11	410
Total other operating expenses	24,205	23,234	50,537	42,802
Income (loss) from operations	9,874	(43,477)	(5,086)	(59,601)
Other income (expense):
Unrealized gain (loss) on investments	6,409	(1,129)	5,431	(1,033)
(Loss) gain on derivative contracts	(14,002)	3,798	(6,109)	1,999
Net foreign exchange (loss) gain	(3,205)	(4,381)	3,431	(7,514)
Other expense	(649)	(1,187)	(1,176)	(2,311)
Interest expense	(11,829)	(11,335)	(28,140)	(22,000)
Total other expense	(23,276)	(14,234)	(26,563)	(30,859)
Loss before income taxes	(13,402)	(57,711)	(31,649)	(90,460)
Income tax (provision) benefit	(626)	11,179	436	18,395
Net loss	(14,028)	(46,532)	(31,213)	(72,065)
Preferred stock dividends	(138)	(138)	(276)	(276)
Loss applicable to common shareholders	(14,166)	(46,670)	(31,489)	(72,341)
Comprehensive loss:
Net loss	(14,028)	(46,532)	(31,213)	(72,065)
Change in fair value of derivative contracts designated as hedge transactions	10,384	3,540	(8,951)	7,799
Comprehensive loss	$ (3,644)	$ (42,992)	$ (40,164)	$ (64,266)
Basic loss per common share after preferred dividends (in dollars per share)	$ (0.03)	$ (0.10)	$ (0.06)	$ (0.15)
Diluted loss per common share after preferred dividends (in dollars per share)	$ (0.03)	$ (0.10)	$ (0.06)	$ (0.15)
Weighted average number of common shares outstanding - basic (in shares)	525,243	486,065	524,218	484,438
Weighted average number of common shares outstanding - diluted (in shares)	525,243	486,065	524,218	484,438
Cash dividends declared per common share (in dollars per share)	$ 0.0025	$ 0.0025	$ 0.0050	$ 0.0050