Segment Information - Schedule of Segment Reporting Information, by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 66,785	$ 59,478
Gain on asset sales	256	651
Operating and maintenance expenses	8,360	6,215
Depreciation, depletion and amortization	4,392	5,100
General and administrative expenses	4,350	4,336
Asset impairments	0	242
Interest expense, net	14,174	17,950
Net income (loss) from continuing operations	35,765	26,286
Net loss from discontinued operations	(46)	(1,948)
Total assets	1,272,171		$ 1,341,647
Corporate and Financing
Segment Reporting Information [Line Items]
Revenues	0	0
Gain on asset sales	0	0
Operating and maintenance expenses	0	0
Depreciation, depletion and amortization	0	0
General and administrative expenses	4,350	4,336
Asset impairments		0
Interest expense, net	14,174	17,950
Net income (loss) from continuing operations	(18,524)	(22,286)
Net loss from discontinued operations	0	0
Coal Royalty and Other
Segment Reporting Information [Line Items]
Revenues	49,502	44,474
Coal Royalty and Other | Operating Segments
Segment Reporting Information [Line Items]
Revenues	55,103	49,857
Gain on asset sales	256	651
Operating and maintenance expenses	8,360	6,215
Depreciation, depletion and amortization	4,392	5,100
General and administrative expenses	0	0
Asset impairments		242
Interest expense, net	0	0
Net income (loss) from continuing operations	42,607	38,951
Net loss from discontinued operations	0	0
Soda Ash
Segment Reporting Information [Line Items]
Revenues	11,682	9,621
Soda Ash | Operating Segments
Segment Reporting Information [Line Items]
Revenues	11,682	9,621
Gain on asset sales	0	0
Operating and maintenance expenses	0	0
Depreciation, depletion and amortization	0	0
General and administrative expenses	0	0
Asset impairments		0
Interest expense, net	0	0
Net income (loss) from continuing operations	11,682	9,621
Net loss from discontinued operations	0	$ 0
Continuing Operations
Segment Reporting Information [Line Items]
Total assets	1,271,173		1,340,654
Continuing Operations | Corporate and Financing
Segment Reporting Information [Line Items]
Total assets	19,852		106,923
Continuing Operations | Coal Royalty and Other | Operating Segments
Segment Reporting Information [Line Items]
Total assets	1,001,385		986,680
Continuing Operations | Soda Ash | Operating Segments
Segment Reporting Information [Line Items]
Total assets	249,936		247,051
Discontinued Operations
Segment Reporting Information [Line Items]
Total assets	998		993
Discontinued Operations | Corporate and Financing
Segment Reporting Information [Line Items]
Total assets	0		0
Discontinued Operations | Coal Royalty and Other | Operating Segments
Segment Reporting Information [Line Items]
Total assets	0		0
Discontinued Operations | Soda Ash | Operating Segments
Segment Reporting Information [Line Items]
Total assets	$ 0		$ 0