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LEASES - As Lessor - Unbilled Straight-Line Rent (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Unbilled Straight-Line Rent      
Unbilled rent receivable $ 15,438,000 $ 15,037,000  
Period during which amount of unbilled rent receivable is to be billed and received 15 years    
Unbilled straight-line rent receivables written off related to property sold $ 365,000 182,000 $ 45,000
Unbilled rent payables $ 801,000 662,000  
Period during which amount of unbilled rent payable is to be billed and received 21 years    
Unbilled rent receivable written off of amounts where collectability is less than probable $ 1,094,000    
Unbilled rent straight-line rent receivable written off related to tenant bankruptcy   $ 548,000 $ 1,440,000