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	Aug. 01, 2017	Feb. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Subsidiary, Sale of Stock [Line Items]
Proceeds From Issuance Of Common Stock, net of issuance costs				$ 0	$ 197,607,000	$ 118,328,000
Stock Issued During Period, Value, New Issues					$ 197,607,000	$ 118,328,000
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		$ 122,900,000	$ 108,200,000				$ 74,700,000
Stock Issued During Period, Value, New Issues		$ 125,000,000	$ 110,000,000				$ 76,000,000
Total shares issued under distribution agreement		12,299,032	10,174,992				7,746,052
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