Consolidated Statements of Operations and Comprehensive (Loss) Income - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Sales of products		$ 567,137	$ 577,775	$ 645,957
Cost of sales and other direct production costs		353,994	304,727	338,325
Depreciation, depletion and amortization	[1]	134,044	120,599	123,631
Total cost of sales		488,038	425,326	461,956
Gross profit		79,099	152,449	184,001
Other operating expenses:
General and administrative		36,542	35,611	45,040
Exploration		35,695	23,510	14,720
Pre-development		4,887	5,448	3,137
Research and development		5,441	3,276	243
Provision for closed operations and environmental matters		6,119	6,701	5,721
Other operating expense		1,596	2,513	3,153
(Gain) loss on disposition of properties, plants, equipment and mineral interests		(2,793)	(6,042)	(147)
Suspension-related costs		20,693	21,301
Acquisition costs		10,045	25	2,695
Total other operating expense		118,225	92,343	74,562
Income (loss) from operations	[1]	(39,126)	60,106	109,439
Other income (expense):
Gain (loss) on derivative contracts		40,253	(21,250)	4,423
Loss on disposition of investments		(34)	(166)
Unrealized loss on investments		(2,816)	(251)	(177)
Net foreign exchange (loss) gain		10,310	(9,680)	(2,737)
Other (expense) income		(907)	1,692	507
Interest expense, net of amount capitalized		(40,944)	(38,012)	(21,796)
Total other income (expense):		5,862	(67,663)	(19,780)
(Loss) income before income taxes		(33,264)	(7,557)	89,659
Income tax benefit (provision)		6,701	(20,963)	(28,090)
Net income (loss)		(26,563)	(28,520)	61,569
Preferred stock dividends		(552)	(552)	(552)
(Loss) income applicable to common stockholders		(27,115)	(29,072)	61,017
Comprehensive (loss) income:
Net income (loss)		(26,563)	(28,520)	61,569
Unrealized gain (loss) and amortization of prior service on pension plans		(1,550)	(1,326)	676
Unrealized gain (loss) on derivative contracts designated as hedge transactions		(13,814)	10,290	(5,260)
Unrealized holding gains (losses) on investments		(2,443)	2,098	1,613
Reclassification of disposition or impairment of investments included in net (loss) income			167	1,000
Total change in accumulated other comprehensive (loss) income, net		(17,807)	11,229	(1,971)
Comprehensive (loss) income		$ (44,370)	$ (17,291)	$ 59,598
Basic (loss) income per common share after preferred dividends (in dollars per share)		$ (0.06)	$ (0.07)	$ 0.16
Diluted (loss) income per common share after preferred dividends (in dollars per share)		$ (0.06)	$ (0.07)	$ 0.16
Weighted average number of common shares outstanding – basic (in shares)		433,419	397,394	386,416
Weighted average number of common shares outstanding – diluted (in shares)		433,419	397,394	389,322

[1] Amounts reported as of and for the years ended December 31, 2017 and 2016 have been revised. See Note 2 for more information.