Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Sales of products	$ 147,259	$ 134,279	$ 286,968	$ 276,823
Cost of sales and other direct production costs	80,440	77,503	153,309	156,179
Depreciation, depletion and amortization	31,817	25,569	59,871	54,521
Total cost of sales	112,257	103,072	213,180	210,700
Gross profit	35,002	31,207	73,788	66,123
Other operating expenses:
General and administrative	9,787	10,309	17,522	19,515
Exploration	7,838	5,853	15,198	10,367
Pre-development	1,415	1,052	2,420	2,304
Research and development	2,337	312	3,773	995
Other operating expense	638	699	1,153	1,362
Provision for closed operations and environmental matters	1,420	985	2,682	2,104
Lucky Friday suspension-related costs	6,801	8,024	11,818	9,605
Acquisition costs	1,010	(2)	3,517	25
Total other operating expenses	31,246	27,232	58,083	46,277
Income from operations	3,756	3,975	15,705	19,846
Other income (expense):
Loss on disposition of investments				(167)
Unrealized (loss) gain on investments	(564)	(276)	(254)	51
Gain (loss) on derivative contracts	16,804	2,487	20,811	(5,322)
Net foreign exchange gain (loss)	2,476	(3,883)	5,068	(6,145)
Interest income and other income and expense	108	319	52	644
Interest expense, net of amount capitalized	(10,079)	(10,543)	(19,873)	(19,065)
Total other income (expense)	8,745	(11,896)	5,804	(30,004)
Income (loss) before income taxes	12,501	(7,921)	21,509	(10,158)
Income tax (provision) benefit	(427)	(16,095)	(1,195)	12,976
Net income (loss)	12,074	(24,016)	20,314	2,818
Preferred stock dividends	(138)	(138)	(276)	(276)
Income (loss) applicable to common shareholders	11,936	(24,154)	20,038	2,542
Comprehensive income (loss):
Net income	12,074	(24,016)	20,314	2,818
Unrealized loss and amortization of prior service on pension plans		(16)		16
Change in fair value of derivative contracts designated as hedge transactions	(5,203)	2,047	(7,276)	5,308
Reclassification of loss on disposition or impairment of marketable securities included in net (loss) income				167
Unrealized holding (losses) gains on investments	41	847	10	591
Comprehensive income (loss)	$ 6,912	$ (21,138)	$ 13,048	$ 8,900
Basic income (loss) per common share after preferred dividends (in dollars per share)	$ 0.03	$ (0.06)	$ 0.05	$ 0.01
Diluted income (loss) per common share after preferred dividends (in dollars per share)	$ 0.03	$ (0.06)	$ 0.05	$ 0.01
Weighted average number of common shares outstanding - basic (in shares)	400,619	396,178	399,972	395,774
Weighted average number of common shares outstanding - diluted (in shares)	403,610	396,178	402,873	399,236
Cash dividends declared per common share (in dollars per share)	$ 0.0025	$ 0.0025	$ 0.005	$ 0.005