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SHAREHOLDERS' EQUITY AND PARTNERS' CAPITAL (At-The-Market Equity Program) (Details) - USD ($)
 12 Months Ended 58 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Mar. 01, 2013
Class of Stock [Line Items]          
Common stock offering, maximum aggregate price (up to)         $ 300,000,000
Commission to sales agent, percent         2.00%
Issuance of shares of stock (in shares) 348,809 335,417 278,093    
Common stock offering, maximum remaining aggregate price $ 88,507,000     $ 88,507,000  
Common Stock          
Class of Stock [Line Items]          
Proceeds from Issuance of Common Stock, Gross 211,493,000        
Issuance of shares of stock (in shares)       8,419,298  
Net proceeds $ 209,596,000        
At The Market Stock Sales          
Class of Stock [Line Items]          
Proceeds from sale of common stock weighted average price per share (in dollars per share) $ 25.12