Concentration Risk (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) tenant	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) tenant	Jun. 30, 2017 USD ($)
Concentration Risk
Number of significant tenants | tenant	1		1
Rental income	$ 10,057	$ 10,471	$ 19,998	$ 17,274
Approximate total acres | Geographic concentration
Concentration Risk
Concentration risk (as a percent)			100.00%	100.00%
Approximate total acres | Geographic concentration | Corn Belt
Concentration Risk
Concentration risk (as a percent)			28.70%	30.80%
Approximate total acres | Geographic concentration | Delta and South
Concentration Risk
Concentration risk (as a percent)			17.60%	18.80%
Approximate total acres | Geographic concentration | High Plains
Concentration Risk
Concentration risk (as a percent)			19.00%	20.40%
Approximate total acres | Geographic concentration | Southeast
Concentration Risk
Concentration risk (as a percent)			27.70%	25.80%
Approximate total acres | Geographic concentration | West Coast
Concentration Risk
Concentration risk (as a percent)			7.00%	4.20%
Rental income | Tenant concentration | Tenant A
Concentration Risk
Rental income	$ 1,111	$ 1,201	$ 2,215	$ 1,978
Concentration risk (as a percent)	11.00%	11.30%	11.10%	11.50%
Rental income | Geographic concentration
Concentration Risk
Concentration risk (as a percent)	100.00%	100.00%	100.00%	100.00%
Rental income | Geographic concentration | Corn Belt
Concentration Risk
Concentration risk (as a percent)	36.60%	32.70%	36.80%	36.70%
Rental income | Geographic concentration | Delta and South
Concentration Risk
Concentration risk (as a percent)	10.30%	14.20%	10.40%	13.30%
Rental income | Geographic concentration | High Plains
Concentration Risk
Concentration risk (as a percent)	8.60%	8.90%	8.50%	9.70%
Rental income | Geographic concentration | Southeast
Concentration Risk
Concentration risk (as a percent)	24.10%	22.30%	24.50%	20.20%
Rental income | Geographic concentration | West Coast
Concentration Risk
Concentration risk (as a percent)	20.40%	21.90%	19.80%	20.10%