Discontinued Operations - Statements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (loss) from discontinued operations	$ 550	$ 364
Cash provided by (used in) operating activities, discontinued operations	(2,690)	11,335
Cash provided by (used in) investing activities, discontinued operations	(28,470)	(388)
Held-for-sale
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue	12,346	20,584
Costs applicable to sales	12,269	18,222
Amortization	0	1,411
General and administrative	41	8
Pre-development, reclamation, and other	265	744
Interest expense, net of capitalized interest	(3)	(6)
Disposal Group, Including Discontinued Operations, Other Income (Expense)	(260)	340
Pretax profit (loss) on discontinued operations related to major classes of pretax profit (loss)	(492)	533
Pretax gain on the disposal of the discontinued operation	1,525	0
Total pretax gain or loss on discontinued operations	1,033	533
Income and mining tax (expense) benefit	(483)	(169)
Cash provided by (used in) operating activities, discontinued operations	(2,690)	11,335
Cash provided by (used in) investing activities, discontinued operations	$ (28,470)	$ (388)