Discontinued Operations - Statements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (loss) from discontinued operations	$ 0	$ (960)	$ 550	$ (596)
Cash provided by (used in) operating activities, discontinued operations	0	5,175	(2,690)	16,510
Cash provided by (used in) investing activities, discontinued operations	0	(375)	(28,470)	(763)
Held-for-sale
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue	0	23,814	12,346	44,398
Costs applicable to sales	0	23,392	12,269	41,614
Amortization	0	2,212	0	3,623
General and administrative	0	17	41	25
Pre-development, reclamation, and other	0	281	265	1,025
Interest expense, net of capitalized interest	0	(6)	(3)	(12)
Disposal Group, Including Discontinued Operations, Other Income (Expense)	0	161	(260)	501
Pretax profit (loss) on discontinued operations related to major classes of pretax profit (loss)	0	(1,933)	(492)	(1,400)
Pretax gain on the disposal of the discontinued operation	0	0	1,525	0
Total pretax gain or loss on discontinued operations	0	(1,933)	1,033	(1,400)
Income and mining tax (expense) benefit	$ 0	973	(483)	804
Cash provided by (used in) operating activities, discontinued operations		5,175	(2,690)	16,510
Cash provided by (used in) investing activities, discontinued operations		$ (375)	$ (28,470)	$ (763)