Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue		$ 143,855	$ 148,795	$ 169,987	$ 163,267	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 625,904	$ 709,598	$ 571,897
COSTS AND EXPENSES
Amortization		37,053	31,184	29,459	30,777	44,722	32,401	30,733	38,693	128,473	146,549	116,528
General and administrative										31,345	33,616	29,275
Impairment of Long-Lived Assets Held-for-use										0	0	4,446
Pre-development, reclamation, and other						15,106	12,374	11,110	13,962	20,043	18,936	14,411
Total costs and expenses										646,208	669,672	512,965
OTHER INCOME (EXPENSE), NET
Loss on debt extinguishments										0	(9,342)	(21,365)
Fair value adjustments, net, pretax										3,638	(864)	(11,581)
Interest expense, net of capitalized interest										(24,364)	(16,440)	(36,896)
Other, net										(24,705)	26,643	98
Total other income (expense), net										(45,431)	(3)	(69,744)
Total										(65,735)	39,923	(10,812)
Income and mining tax (expense) benefit										16,780	(28,998)	33,247
Income (loss) from continuing operations		468	(53,044)	2,930	691	14,349	(11,728)	(9,995)	18,299	(48,955)	10,925	22,435
Income (loss) from discontinued operations		0	0	0	550	(6,724)	(4,924)	(960)	364	550	(12,244)	32,917
NET INCOME (LOSS)		$ 468	$ (53,044)	$ 2,930	$ 1,241	$ 7,625	$ (16,652)	$ (10,955)	$ 18,663	(48,405)	(1,319)	55,352
OTHER COMPREHENSIVE INCOME (LOSS), Net of Tax:
Unrealized gain (loss) on debt and equity securities										26	3,227	3,222
Reclassification adjustments for impairment of equity securities										0	426	703
Reclassification adjustments for realized (gain) loss on sale of equity securities										0	1,354	(2,691)
Other comprehensive income (loss)										26	5,007	1,234
COMPREHENSIVE INCOME (LOSS)										$ (48,379)	$ 3,688	$ 56,586
Basic EPS
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.00	$ (0.290)	$ 0.020	$ 0.000	$ 0.08	$ (0.060)	$ (0.050)	$ 0.100	$ (0.260)	$ 0.060	$ 0.140
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share		0.00	0.000	0.000	0.000	(0.04)	(0.030)	(0.010)	0.000	0.000	(0.070)	0.210
Earnings Per Share, Basic	[1]	0.000	(0.290)	0.020	0.000	0.040	(0.090)	(0.060)	0.100	(0.260)	(0.010)	0.350
Diluted EPS
Income (Loss) from Continuing Operations, Per Diluted Share		0.000	(0.290)	0.020	0.000	0.080	(0.060)	(0.050)	0.100	(0.260)	0.060	0.140
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share		0.000	0.000	0.000	0.000	(0.040)	(0.030)	(0.010)	0.000	0.000	(0.070)	0.200
Earnings Per Share, Diluted	[1]	$ 0.000	$ (0.290)	$ 0.020	$ 0.000	$ 0.040	$ (0.090)	$ (0.060)	$ 0.100	$ (0.260)	$ (0.010)	$ 0.340
Product [Member]
COSTS AND EXPENSES
Costs applicable to sales	[2]	$ 116,507	$ 116,857	$ 108,246	$ 99,340	$ 121,982	$ 101,559	$ 102,229	$ 114,490	$ 440,950	$ 440,260	$ 335,375
Mineral, Exploration [Member]
COSTS AND EXPENSES
Costs applicable to sales		$ 4,128	$ 8,157	$ 6,429	$ 6,683	$ 7,454	$ 9,792	$ 7,813	$ 5,252	$ 25,397	$ 30,311	12,930
General and administrative												$ 29,275

[1] Due to rounding, the sum of net income per share from continuing operations and discontinued operations may not equal net income per share.

[2] Excludes amortization.