Concentration Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk
Rental income recognized	$ 48,119	$ 51,185	$ 42,956
Approximate total acres | Geographic concentration
Concentration Risk
Concentration risk (as a percent)	100.00%	100.00%	100.00%
Approximate total acres | Geographic concentration | Alabama
Concentration Risk
Concentration risk (as a percent)	0.40%	0.30%	0.40%
Approximate total acres | Geographic concentration | Arkansas
Concentration Risk
Concentration risk (as a percent)	9.10%	8.30%	9.10%
Approximate total acres | Geographic concentration | California
Concentration Risk
Concentration risk (as a percent)	7.30%	7.20%	7.20%
Approximate total acres | Geographic concentration | Colorado
Concentration Risk
Concentration risk (as a percent)	15.50%	14.90%	15.10%
Approximate total acres | Geographic concentration | Florida
Concentration Risk
Concentration risk (as a percent)	3.00%	4.60%	4.60%
Approximate total acres | Geographic concentration | Georgia
Concentration Risk
Concentration risk (as a percent)	3.40%	3.30%	3.30%
Approximate total acres | Geographic concentration | Illinois
Concentration Risk
Concentration risk (as a percent)	24.50%	24.10%	25.50%
Approximate total acres | Geographic concentration | Kansas
Concentration Risk
Concentration risk (as a percent)	1.20%	1.20%	1.20%
Approximate total acres | Geographic concentration | Louisiana
Concentration Risk
Concentration risk (as a percent)	5.30%	5.20%	5.90%
Approximate total acres | Geographic concentration | Michigan
Concentration Risk
Concentration risk (as a percent)	0.30%	1.40%	1.40%
Approximate total acres | Geographic concentration | Mississippi
Concentration Risk
Concentration risk (as a percent)	3.10%	3.00%	3.10%
Approximate total acres | Geographic concentration | North Carolina
Concentration Risk
Concentration risk (as a percent)	10.50%	10.30%	6.90%
Approximate total acres | Geographic concentration | Nebraska
Concentration Risk
Concentration risk (as a percent)	3.80%	3.70%	3.70%
Approximate total acres | Geographic concentration | South Carolina
Concentration Risk
Concentration risk (as a percent)	9.50%	9.40%	9.10%
Approximate total acres | Geographic concentration | South Dakota
Concentration Risk
Concentration risk (as a percent)	1.00%	1.00%	1.10%
Approximate total acres | Geographic concentration | Texas
Concentration Risk
Concentration risk (as a percent)	1.30%	1.30%	1.70%
Approximate total acres | Geographic concentration | Virginia
Concentration Risk
Concentration risk (as a percent)	0.80%	0.80%	0.70%
Rental income | Tenant concentration | Tenant A
Concentration Risk
Rental income recognized	$ 5,905	$ 6,996	$ 7,041
Concentration risk (as a percent)	12.20%	13.60%	16.40%
Rental income | Tenant concentration | Tenant B
Concentration Risk
Rental income recognized	$ 7,056	$ 6,957
Concentration risk (as a percent)	14.60%	13.60%
Rental income | Geographic concentration
Concentration Risk
Concentration risk (as a percent)	100.00%	100.00%	100.00%
Rental income | Geographic concentration | Alabama
Concentration Risk
Concentration risk (as a percent)	0.20%	0.40%	0.20%
Rental income | Geographic concentration | Arkansas
Concentration Risk
Concentration risk (as a percent)	4.40%	4.00%	4.70%
Rental income | Geographic concentration | California
Concentration Risk
Concentration risk (as a percent)	34.60%	36.80%	27.80%
Rental income | Geographic concentration | Colorado
Concentration Risk
Concentration risk (as a percent)	5.40%	4.40%	6.40%
Rental income | Geographic concentration | Florida
Concentration Risk
Concentration risk (as a percent)	2.10%	1.70%	3.10%
Rental income | Geographic concentration | Georgia
Concentration Risk
Concentration risk (as a percent)	2.10%	2.10%	4.00%
Rental income | Geographic concentration | Illinois
Concentration Risk
Concentration risk (as a percent)	24.50%	24.30%	28.80%
Rental income | Geographic concentration | Kansas
Concentration Risk
Concentration risk (as a percent)	0.30%	0.30%	0.40%
Rental income | Geographic concentration | Louisiana
Concentration Risk
Concentration risk (as a percent)	3.10%	2.80%	3.30%
Rental income | Geographic concentration | Michigan
Concentration Risk
Concentration risk (as a percent)	0.80%	1.70%	1.60%
Rental income | Geographic concentration | Mississippi
Concentration Risk
Concentration risk (as a percent)	1.70%	1.70%	2.00%
Rental income | Geographic concentration | North Carolina
Concentration Risk
Concentration risk (as a percent)	7.40%	7.10%	4.30%
Rental income | Geographic concentration | Nebraska
Concentration Risk
Concentration risk (as a percent)	3.30%	3.00%	3.60%
Rental income | Geographic concentration | South Carolina
Concentration Risk
Concentration risk (as a percent)	8.20%	7.90%	7.70%
Rental income | Geographic concentration | South Dakota
Concentration Risk
Concentration risk (as a percent)	0.80%	0.70%	0.60%
Rental income | Geographic concentration | Texas
Concentration Risk
Concentration risk (as a percent)	0.70%	0.80%	1.10%
Rental income | Geographic concentration | Virginia
Concentration Risk
Concentration risk (as a percent)	0.40%	0.40%	0.40%