Condensed Consolidated Statements of Operations and Comprehensive (Loss) Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Sales of products	$ 134,172	$ 147,259	$ 286,789	$ 286,968
Cost of sales and other direct production costs	104,938	80,440	215,324	153,309
Depreciation, depletion and amortization	49,477	31,817	88,264	59,871
Total cost of sales	154,415	112,257	303,588	213,180
Gross (loss) profit	(20,243)	35,002	(16,799)	73,788
Other operating expenses:
General and administrative	8,918	9,787	18,877	17,522
Exploration	4,346	7,838	8,748	15,198
Pre-development	798	1,415	1,654	2,420
Research and development	158	2,337	561	3,773
Other operating expense	657	674	1,244	1,319
Loss (gain) on disposition or impairment of properties, plants, equipment and mineral interests	4,642	(36)	4,642	(166)
Provision for closed operations and environmental matters	1,052	1,420	1,622	2,682
Suspension-related costs	2,266	6,801	5,044	11,818
Acquisition costs	397	1,010	410	3,517
Total other operating expenses	23,234	31,246	42,802	58,083
(Loss) income from operations	(43,477)	3,756	(59,601)	15,705
Other income (expense):
Unrealized loss on investments	(1,129)	(564)	(1,033)	(254)
Gain on derivative contracts	3,798	16,804	1,999	20,811
Net foreign exchange gain (loss)	(4,381)	2,476	(7,514)	5,068
Other (expense) income	(1,187)	108	(2,311)	52
Interest expense	(11,335)	(10,079)	(22,000)	(19,873)
Total other (expense) income	(14,234)	8,745	(30,859)	5,804
(Loss) income before income taxes	(57,711)	12,501	(90,460)	21,509
Income tax benefit (provision)	11,179	(427)	18,395	(1,195)
Net income (loss)	(46,532)	12,074	(72,065)	20,314
Preferred stock dividends	(138)	(138)	(276)	(276)
(Loss) income applicable to common shareholders	(46,670)	11,936	(72,341)	20,038
Comprehensive (loss) income:
Net (loss) income	(46,532)	12,074	(72,065)	20,314
Change in fair value of derivative contracts designated as hedge transactions	3,540	(5,203)	7,799	(7,276)
Unrealized holding (losses) gains on investments	0	41	0	10
Comprehensive (loss) income	$ (42,992)	$ 6,912	$ (64,266)	$ 13,048
Basic (loss) income per common share after preferred dividends (in dollars per share)	$ (0.10)	$ 0.03	$ (0.15)	$ 0.05
Diluted (loss) income per common share after preferred dividends (in dollars per share)	$ (0.10)	$ 0.03	$ (0.15)	$ 0.05
Weighted average number of common shares outstanding - basic (in shares)	486,065	400,619	484,438	399,972
Weighted average number of common shares outstanding - diluted (in shares)	486,065	403,610	484,438	402,873
Cash dividends declared per common share (in dollars per share)	$ 0.0025	$ 0.0025	$ 0.0050	$ 0.0050