RENTALS UNDER OPERATING LEASES Concentration Risk (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	48.40%	52.40%	48.30%	54.00%
Property | Seattle East Industrial, Redmond, WA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	14.00%		13.70%
Property | Providence Square, Marietta, GA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	12.80%		14.10%
Property | Flats at Carrs Hill, Athens, GA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	11.30%	14.80%	10.70%	14.40%
Property | Loudoun Gateway, Sterling, VA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	10.30%	12.60%	9.80%	13.20%
Property | Allied Drive, Dedham, MA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		12.90%		12.80%
Property | Elston Plaza, Chicago, IL | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		12.10%		13.60%
Tenant | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	24.30%	24.00%	23.50%	24.60%
Tenant | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			26.20%	13.60%
Tenant | FedEx Ground - Seattle East Industrial | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	14.00%		13.70%
Tenant | FedEx Ground - Seattle East Industrial | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			14.80%	0.00%
Tenant | Orbital ATK Inc. - Loudoun Gateway | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	10.30%	12.60%	9.80%	13.20%
Tenant | Orbital ATK Inc. - Loudoun Gateway | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			11.40%	13.60%
Tenant | New England Baptist Hospital-Allied Drive | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		11.40%		11.40%