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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
$ / shares
Sep. 30, 2016
$ / shares
Sep. 30, 2017
segment
property
$ / shares
Sep. 30, 2016
$ / shares
Dec. 31, 2016
property
Accounting Policies [Abstract]          
Number of reportable segments | segment     1    
Distributions declared per common share (in dollars per share) | $ / shares $ 0.09452055 $ 0.0942623 $ 0.28047945 $ 0.2807377  
Real Estate [Line Items]          
Number of real estate properties disposed         0
Office Properties          
Real Estate [Line Items]          
Number of real estate properties disposed     1