Discontinued Operations - Statements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (loss) from discontinued operations	$ 0	$ (4,924)	$ 550	$ (5,520)
Cash provided by (used in) operating activities, discontinued operations	0	(7,877)	(2,690)	8,633
Cash provided by (used in) investing activities, discontinued operations	0	(412)	(28,470)	(1,175)
Held-for-sale
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue	0	16,043	12,346	60,441
Costs applicable to sales	0	17,365	12,269	58,979
Amortization	0	1,430	0	5,053
General and administrative	0	67	41	92
Exploration	0	23	0	23
Pre-development, reclamation, and other	0	2,931	265	3,956
Interest expense, net of capitalized interest	0	(11)	(3)	(23)
Disposal Group, Including Discontinued Operations, Other Income (Expense)	0	804	(260)	1,305
Pretax profit (loss) on discontinued operations related to major classes of pretax profit (loss)	0	(4,980)	(492)	(6,380)
Pretax gain on the disposal of the discontinued operation	0	0	1,525	0
Total pretax gain or loss on discontinued operations	0	(4,980)	1,033	(6,380)
Income and mining tax (expense) benefit	$ 0	56	(483)	860
Cash provided by (used in) operating activities, discontinued operations		(7,877)	(2,690)	8,633
Cash provided by (used in) investing activities, discontinued operations		$ (412)	$ (28,470)	$ (1,175)