Discontinued Operations - Discontinued operations results of operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lease revenue	$ 3,591	$ 3,581	$ 10,636	$ 10,796
Depreciation, depletion and amortization	1,438	1,431	4,406	4,390
Operating expenses	892	952	2,598	2,744
Property taxes	706	740	2,089	2,206
Total operating profit	1,581	1,358	3,579	4,820
Interest expense	(46)	(129)	(142)	(989)
Income (loss) before income taxes	7,293	2,628	14,908	9,024
Provision for (benefit from) income taxes	2,022	726	4,161	2,529
Income (loss) from discontinued operations	$ 0	$ (13)	$ 0	$ 6,849
Earnings per common share:
Basic earnings per common share Discontinued operations	$ 0.00	$ 0.00	$ 0.00	$ 0.69
Diluted earnings per common share discontinued operations	$ 0.00	$ 0.00	$ 0.00	$ 0.69
Discontinued Operations, Disposed of by Sale [Member]
Lease revenue		$ 0		$ 460
Depreciation, depletion and amortization		(24)		17
Operating expenses		12		246
Property taxes		0		46
Management company indirect		0		0
Corporate expenses		0		0
Total cost of operations		(12)		309
Total operating profit		12		151
Interest expense		0		0
Gain (loss) on sale of buildings		(30)		9,238
Income (loss) before income taxes		(18)		9,389
Provision for (benefit from) income taxes		(5)		2,540
Income (loss) from discontinued operations		$ (13)		$ 6,849
Earnings per common share:
Basic earnings per common share Discontinued operations		$ 0.00		$ 0.69
Diluted earnings per common share discontinued operations		$ 0.00		$ 0.69