REAL ESTATE PROPERTIES - Summary of Multi-family Properties (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018 USD ($) property property_unit	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) property property_unit	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	36								36
Number of Units at September 30, 2018 | property_unit	10,121								10,121
Total revenues | $	$ 31,481	$ 30,154	$ 29,651	$ 28,349	$ 28,387	$ 26,861	$ 24,883	$ 25,640	$ 119,635	$ 105,771	$ 98,521
Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	36								36
Number of Units at September 30, 2018 | property_unit	10,121								10,121
Total revenues | $									$ 117,339
% of 2018 Revenues									100.00%
Texas | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	11								11
Number of Units at September 30, 2018 | property_unit	3,096								3,096
Total revenues | $									$ 36,358
% of 2018 Revenues									31.00%
Georgia | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	5								5
Number of Units at September 30, 2018 | property_unit	1,545								1,545
Total revenues | $									$ 16,761
% of 2018 Revenues									14.30%
Florida | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	4								4
Number of Units at September 30, 2018 | property_unit	1,248								1,248
Total revenues | $									$ 15,832
% of 2018 Revenues									13.50%
Missouri | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	4								4
Number of Units at September 30, 2018 | property_unit	775								775
Total revenues | $									$ 10,622
% of 2018 Revenues									9.10%
South Carolina | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	3								3
Number of Units at September 30, 2018 | property_unit	683								683
Total revenues | $									$ 8,717
% of 2018 Revenues									7.40%
Tennessee | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	2								2
Number of Units at September 30, 2018 | property_unit	702								702
Total revenues | $									$ 4,215
% of 2018 Revenues									3.60%
Alabama | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	2								2
Number of Units at September 30, 2018 | property_unit	412								412
Total revenues | $									$ 6,135
% of 2018 Revenues									5.20%
Mississippi | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	2								2
Number of Units at September 30, 2018 | property_unit	776								776
Total revenues | $									$ 8,617
% of 2018 Revenues									7.30%
Indiana | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	1								1
Number of Units at September 30, 2018 | property_unit	400								400
Total revenues | $									$ 3,689
% of 2018 Revenues									3.10%
Ohio | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	1								1
Number of Units at September 30, 2018 | property_unit	264								264
Total revenues | $									$ 2,804
% of 2018 Revenues									2.40%
Virginia | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Properties at September 30, 2018 | property	1								1
Number of Units at September 30, 2018 | property_unit	220								220
Total revenues | $									$ 3,589
% of 2018 Revenues									3.10%
Melbourne, FL | Geographic concentration | Multi-family residential | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Units at September 30, 2018 | property_unit	1,368								1,368
Total revenues | $									$ 5,815