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SHAREHOLDERS' EQUITY AND PARTNERS' CAPITAL (Common Stock Sold) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Mar. 01, 2013
Class of Stock [Line Items]            
Number of Shares Settled     278,093 246,168 8,772,114  
Common Stock            
Class of Stock [Line Items]            
Common stock offering, maximum aggregate price           $ 300,000,000
Common stock offering, maximum commission fee (percent)           2.00%
Number of Shares Settled 6,530,193 1,889,105 8,419,298   8,419,298  
Gross Proceeds $ 167,034,000 $ 44,459,000     $ 211,493,000  
Net Proceeds $ 165,692,000 $ 43,904,000     $ 209,596,000  
Weighted-average Sales Price (in dollars per share) $ 25.58 $ 23.53     $ 25.12  
Common stock offering, maximum remaining aggregate price     $ 88,507,000