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ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES - Capital Raising Activities (Narrative) (Details) - USD ($)
2 Months Ended 5 Months Ended 7 Months Ended 12 Months Ended 17 Months Ended
Feb. 20, 2013
Dec. 31, 2017
Aug. 01, 2017
Feb. 22, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jul. 28, 2016
Subsidiary, Sale of Stock [Line Items]                
Proceeds From Issuance Of Common Stock, net of issuance costs         $ 197,607,000 $ 118,328,000 $ 83,275,000  
Stock Issued During Period, Value, New Issues         $ 197,607,000 $ 118,328,000 $ 83,275,000  
Initial Public Offering [Member]                
Subsidiary, Sale of Stock [Line Items]                
Stock Issued During Period, Value, New Issues $ 35,400,000              
Total shares issued under distribution agreement 2,400,000              
At the Market Offering Program [Member]                
Subsidiary, Sale of Stock [Line Items]                
Proceeds From Issuance Of Common Stock, net of issuance costs   $ 74,700,000 $ 122,900,000 $ 108,200,000       $ 93,200,000
Stock Issued During Period, Value, New Issues   $ 76,000,000 $ 125,000,000 $ 110,000,000       $ 95,400,000
Total shares issued under distribution agreement   0 12,299,032 10,174,992       7,218,766
At the Market Offering Program [Member] | Maximum [Member]                
Subsidiary, Sale of Stock [Line Items]                
Stock Issued During Period, Value, New Issues   $ 125,000,000 $ 125,000,000 $ 125,000,000       $ 100,000,000,000