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STOCKHOLDERS' EQUITY - Common Stock Dividend Distributions (Details) - USD ($)
 12 Months Ended
Mar. 12, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stockholders' Equity        
Cash distributions declared per share of common stock (in dollars per share)   $ 1.74 $ 1.66 $ 1.58
Cash dividend declared   $ 32,391,000 $ 29,136,000 $ 26,178,000
Dividend Reinvestment Plan        
Common shares issued under Dividend Reinvestment Plan   198,000 142,000 197,000
Shares Issued Through Equity Offering Program        
Number of shares sold (in shares)   231,000 1,079,862  
Proceeds from sale of shares, net of commission and before offering costs   $ 5,758,000 $ 25,947,000  
Payment of commissions on sale of shares   58,000 262,000  
Payment of Offering Costs on Sale of Shares   $ 188,000 $ 160,000  
Maximum        
Dividend Reinvestment Plan        
Discount rate (as a percent)   5.00%    
Dividend Subsequent to Year End        
Stockholders' Equity        
Cash distributions declared per share of common stock (in dollars per share) $ 0.45      
Cash dividend declared $ 8,581,000