RENTALS UNDER OPERATING LEASES Concentration Risk (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Property | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	39.60%	64.60%	38.70%	64.80%
Property | Providence Square, Marietta, GA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	15.60%		14.80%
Property | Seattle East Industrial, Redmond, WA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	13.60%		13.50%
Property | Flats at Carrs Hill, Athens, GA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	10.40%	14.50%	10.40%	14.10%
Property | Elston Plaza, Chicago, IL | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		14.50%		14.40%
Property | Loudoun Gateway, Sterling, VA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		12.80%		13.50%
Property | Allied Drive, Dedham, MA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		12.80%		12.80%
Property | Terra Nova Plaza, Chula Vista, CA | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		10.00%		10.00%
Tenant | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	13.60%	24.20%	13.50%	24.90%
Tenant | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			26.80%	16.80%
FedEx Ground - Seattle East Industrial | Tenant | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage	13.60%		13.50%
FedEx Ground - Seattle East Industrial | Tenant | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			15.20%	0.00%
Orbital ATK Inc. - Loudoun Gateway | Tenant | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		12.80%		13.50%
Orbital ATK Inc. - Loudoun Gateway | Tenant | Percent of in-place annualized base rental revenues as of
Lessor, Lease, Description [Line Items]
Concentration risk percentage			11.60%	16.80%
New England Baptist Hospital-Allied Drive | Tenant | Property-related income
Lessor, Lease, Description [Line Items]
Concentration risk percentage		11.40%		11.40%