REAL ESTATE OPERATIONS - Subsurface Interests (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) a county	Sep. 30, 2019 USD ($) a	Sep. 30, 2020 USD ($) a county	Sep. 30, 2019 USD ($) a
Subsurface interests
Revenue from contract with customer, including assessed tax	$ 544,000	$ 214,000	$ 631,000	$ 709,000
Over Lease Term	$ 12,933,000	10,261,000	$ 35,409,000	31,361,000
Subsurface Interests
Subsurface interests
Area of land (in acres) | a	345		345
Revenue from contract with customer, including assessed tax	$ 412,000		$ 412,000	0
Over Lease Term		$ 186,000	$ 0	$ 586,000
Number of acres with operating oil wells | a	800	800	800	800
Surface land over subsurface interests
Subsurface interests
Area of land (in acres) | a	455,000		455,000
Number of counties in which Subsurface Interests are owned | county	20		20
Revenue recognized for cash payments for the release of surface entry rights			$ 186,000	$ 0
Real Estate Operations | Subsurface Interests
Subsurface interests
Revenue from contract with customer, including assessed tax	$ 12,000	$ 10,000	$ 22,000	$ 39,000