Discontinued Operations - Statements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (loss) from discontinued operations	$ 0	$ 0	$ 5,693	$ 550
Cash provided by (used in) operating activities, discontinued operations	0	0	0	(2,690)
Cash provided by (used in) investing activities, discontinued operations	0	0	0	(28,470)
Held-for-sale
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue	0	0	0	12,346
Costs applicable to sales	0	0	0	12,269
General and administrative	0	0	0	41
Pre-development, reclamation, and other	0	0	0	265
Interest expense, net of capitalized interest	0	0	0	(3)
Disposal Group, Including Discontinued Operations, Other Income (Expense)	0	0	0	(260)
Pretax profit (loss) on discontinued operations related to major classes of pretax profit (loss)	0	0	0	(492)
Pretax gain on the disposal of the discontinued operation	0	0	5,693	1,525
Total pretax gain or loss on discontinued operations	0	0	5,693	1,033
Income and mining tax (expense) benefit	0	0	0	(483)
Income (loss) from discontinued operations	$ 0	$ 0	$ 5,693	550
Cash provided by (used in) operating activities, discontinued operations				(2,700)
Cash provided by (used in) investing activities, discontinued operations				$ (28,500)