Summary of Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Revenue	$ 228,317	$ 229,728	$ 154,249	$ 173,167	$ 195,040	$ 199,469	$ 162,123	$ 154,870	$ 785,461		$ 711,502		$ 625,904
Pre-development, reclamation, and other									55,654		18,421		20,043
Amortization	35,133	32,216	27,876	36,162	48,118	45,678	43,204	41,876	131,387		178,876		128,473
Gross Profit	74,553	84,740	36,358	18,088	291	12,839	(13,029)	(18,656)
Other Cost and Expense, Operating	23,822	22,788	27,291	15,475	263,250	14,486	12,084	13,908
Income (loss) from continuing operations	11,880	26,856	(1,209)	(11,900)	(270,961)	(14,277)	(36,764)	(24,894)	25,627		(346,896)		(48,955)
Income (loss) from discontinued operations	0	0	0	0	0	0	0	5,693	0		5,693		550
NET INCOME (LOSS)									25,627		(341,203)		(48,405)
Cash provided by (used in) operating activities	67,289	79,464	9,947	(7,991)	39,295	41,996	26,435	(15,846)	148,709		91,880		17,418
Capital expenditures	$ 37,393	$ 22,996	$ 16,682	$ 22,208	$ 20,907	$ 30,678	$ 20,749	$ 27,438	$ 99,279		$ 99,772		$ 140,787
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.05	$ 0.11	$ (0.01)	$ (0.05)	$ (1.13)	$ (0.06)	$ (0.18)	$ (0.12)	$ 0.11		$ (1.59)		$ (0.26)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.03	0		0.03		0
Earnings Per Share, Basic	0.05	0.11	(0.01)	(0.05)	(1.13)	(0.06)	(0.18)	(0.09)	0.11	[1]	(1.56)	[1]	(0.26)
Income (Loss) from Continuing Operations, Per Diluted Share	0.05	0.11	(0.01)	(0.05)	(1.13)	(0.06)	(0.18)	(0.12)	0.11		(1.59)		(0.26)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.03	0		0.03		0
Earnings Per Share, Diluted	$ 0.05	$ 0.11	$ (0.01)	$ (0.05)	$ (1.13)	$ (0.06)	$ (0.18)	$ (0.09)	$ 0.11	[1]	$ (1.56)	[1]	$ (0.26)
Accumulated Deficit [Member]
NET INCOME (LOSS)	$ 11,880	$ 26,856	$ (1,209)	$ (11,900)	$ (270,961)	$ (14,277)	$ (36,764)	$ (19,201)	$ 25,627		$ (341,203)		$ (48,405)
Product
Costs applicable to sales	118,631	112,772	90,015	118,917	146,631	140,952	131,948	131,650	440,335	[2]	551,181	[2]	440,950	[2]
Mineral, Exploration
Costs applicable to sales	$ 11,584	$ 12,818	$ 11,855	$ 6,386	$ 7,201	$ 5,893	$ 5,719	$ 3,714	$ 42,643		$ 22,527		$ 25,397

[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.