Condensed Consolidated Statements of Operations and Comprehensive (Loss) Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Sales of products	$ 143,649	$ 140,839	$ 430,617	$ 417,662
Cost of sales and other direct production costs	93,609	68,358	246,918	224,537
Depreciation, depletion and amortization	43,464	29,518	103,335	86,986
Total cost of sales	137,073	97,876	350,253	311,523
Gross profit	6,576	42,963	80,364	106,139
Other operating expenses:
General and administrative	10,327	9,529	27,849	29,044
Exploration	12,411	7,255	27,609	17,622
Pre-development	1,195	1,757	3,615	4,061
Research and development	1,269	1,130	5,042	2,125
Other operating expense	448	134	1,767	1,590
Gain on disposition of properties, plants, equipment and mineral interests	(3,208)	(4,830)	(3,374)	(4,924)
Provision for closed operations and reclamation	1,852	2,940	4,534	5,044
Suspension-related costs	6,519	4,780	18,337	14,385
Acquisition costs	6,139		9,656	25
Total other operating expense	36,952	22,695	95,035	68,972
Income from operations	(30,376)	20,268	(14,671)	37,167
Other income (expense):
Gain (loss) on derivative contracts	19,460	(11,226)	40,271	(16,548)
Loss on disposition of investments	(36)		(36)	(167)
Unrealized (loss) gain on investments	(2,207)	(124)	(2,461)	(73)
Foreign exchange (loss) gain	(2,212)	(4,917)	2,856	(10,258)
Interest income and other (expense) income	(346)	541	(294)	1,185
Interest expense, net of amount capitalized	(10,146)	(9,358)	(30,019)	(28,423)
Total other expense	4,513	(25,084)	10,317	(54,284)
(Loss) income before income taxes	(25,863)	(4,816)	(4,354)	(17,117)
Income tax benefit	2,679	5,130	1,484	17,564
Net income (loss)	(23,184)	314	(2,870)	447
Preferred stock dividends	(138)	(138)	(414)	(414)
Income applicable to common shareholders	(23,322)	176	(3,284)	33
Comprehensive income:
Net (loss) income	(23,184)	314	(2,870)	447
Reclassification of loss on disposition or impairment of marketable securities included in net income				167
Unrealized loss and amortization of prior service on pension plans		(16)
Change in fair value of derivative contracts designated as hedge transactions	3,743	6,760	(3,533)	12,068
Unrealized holding gains on investments	3	892	13	1,483
Comprehensive (loss) income	$ (19,438)	$ 7,950	$ (6,390)	$ 14,165
Basic (loss) income per common share after preferred dividends (in dollars per share)	$ (0.05)		$ (0.01)
Diluted (loss) income per common share after preferred dividends (in dollars per share)	$ (0.05)		$ (0.01)
Weighted average number of common shares outstanding - basic (in shares)	452,636	398,848	417,532	396,809
Weighted average number of common shares outstanding - diluted (in shares)	452,636	401,258	417,532	400,176
Cash dividends declared per common share (in dollars per share)	$ 0.0025	$ 0.0025	$ 0.0075	$ 0.0075