INVESTMENT ACTIVITY (Details)												3 Months Ended						9 Months Ended					12 Months Ended					18 Months Ended
	Sep. 29, 2020 USD ($) shares	Jul. 20, 2020 USD ($)	Dec. 16, 2019 USD ($)	Oct. 07, 2019 USD ($)	Sep. 17, 2019 USD ($)	Sep. 06, 2019 USD ($)	Aug. 08, 2019	Jun. 28, 2019 USD ($)	Jun. 25, 2019 USD ($)	Jun. 06, 2019 USD ($) property	Jun. 05, 2019 USD ($) property	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 item	Dec. 31, 2015 item	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) facility	Sep. 30, 2020 USD ($) property	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) facility	Dec. 31, 2019 USD ($) property	Jun. 30, 2020 USD ($)
Self-storage facilities
Number of facilities, construction completed and opened																							$ 4					$ 1
Ownership interest held by the entity (as a percent)																												100.00%
Total Construction Costs																							$ 136,500,000					$ 45,900,000
Acquisition of noncontrolling interest													$ 2,618,000	$ 27,618,000	$ 10,000,000
HVP III
Self-storage facilities
Percentage of interest acquired										90.00%
New York
Self-storage facilities
Aggregate purchase price		$ 9,500,000
Decrease in right-of-use asset		(5,100,000)
Decrease in operating lease liability		$ (5,000,000.0)
Cash paid for acquisition	$ 1,000,000.0
Total consideration	$ 10,000,000.0
Units issued (in shares) | shares	276,497
Units issued	$ 9,000,000.0
California
Self-storage facilities
Cash paid for acquisition			$ 14,900,000
Issuance of OP Shares			3,600,000
Total consideration			$ 18,500,000
Self storage under construction
Self-storage facilities
Deposit												$ 300,000						$ 300,000	$ 300,000	$ 300,000	$ 300,000
Expected aggregate purchase or sales price												14,700,000						14,700,000	14,700,000	$ 14,700,000	$ 14,700,000
Number of facilities under contract																				2	5
Self storage under construction | New York
Self-storage facilities
Number of facilities under contract | facility																				2
Self storage under construction | Massachusetts
Self-storage facilities
Number of facilities under contract | facility																				1
Self storage under construction | Virginia
Self-storage facilities
Number of facilities under contract | property																					1
Self storage under construction | Pennsylvania
Self-storage facilities
Number of facilities under contract | facility																				1
Self-storage Facility in Waltham, MA
Self-storage facilities
Acquisition of remaining interest in real estate ventures (as a percent)						10.00%
Total consideration						$ 2,600,000
Acquisition, gain (loss) recognized						0
Number of facilities, construction completed and opened																												$ 1
Ownership interest held by the entity (as a percent)																												100.00%
Total Construction Costs																												$ 18,000,000
Acquisition of noncontrolling interest						$ 2,000,000.0
Self Storage Facility in Queens, NY I
Self-storage facilities
Cash paid for acquisition								$ 9,000,000.0
Percentage of interest acquired								10.00%
Acquisition of noncontrolling interest								$ 22,600,000
Repayments of debt								12,400,000
Self Storage Facility in Bayonne, NJ
Self-storage facilities
Number of facilities, construction completed and opened																												$ 1
Ownership interest held by the entity (as a percent)																												100.00%
Total Construction Costs																												$ 25,100,000
Self Storage Facility in Brooklyn, NY I
Self-storage facilities
Cash paid for acquisition									$ 18,500,000
Percentage of interest acquired									10.00%
Ownership interest held by the entity (as a percent)																	90.00%
Number of joint ventures | item																	2
Self Storage Facilities in Brooklyn NY, and Queens NY
Self-storage facilities
Acquisition, gain (loss) recognized								$ 0
Self-storage Facility in Queens, NY
Self-storage facilities
Number of facilities, construction completed and opened																												$ 1
Ownership interest held by the entity (as a percent)																												100.00%
Total Construction Costs																												$ 47,500,000
2020 Acquisitions
Self-storage facilities
Number of self-storage facilities acquired																				3	3
Aggregate purchase price																			74,675,000
2020 Acquisitions | Leases, Acquired-in-Place
Self-storage facilities
Intangible value of the in-place leases												2,500,000						$ 2,500,000	2,500,000	$ 2,500,000	$ 2,500,000
Estimated life																		12 months
Amortization expense												600,000							1,100,000
2020 Acquisitions | Texas
Self-storage facilities
Number of self-storage facilities acquired | property																					1
2020 Acquisitions | Maryland
Self-storage facilities
Number of self-storage facilities acquired | property																					1
2020 Acquisitions | New Jersey
Self-storage facilities
Number of self-storage facilities acquired | property																					1
2019 Acquisitions
Self-storage facilities
Number of self-storage facilities acquired																										29	11
Aggregate purchase price																									$ 246,600,000
Cash paid for acquisition																									14,900,000
Issuance of OP Shares																									3,600,000
2019 Acquisitions | Leases, Acquired-in-Place
Self-storage facilities
Intangible value of the in-place leases																								$ 6,200,000	6,200,000	$ 6,200,000	$ 6,200,000
Estimated life																								12 months
Amortization expense												1,000,000.0	500,000						3,700,000			$ 1,000,000.0
2019 Acquisitions | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										18
Cash paid for acquisition										$ 120,000,000.0
Percentage of interest acquired										90.00%
Acquisition of remaining interest in real estate ventures (as a percent)										10.00%
Total consideration										$ 128,300,000
Amount of escrowed proceeds from sale of assets										8,300,000
Acquisition, assets recorded										137,500,000
Acquistion, previously held equity interest										10,600,000
Acquisition, cash acquired										1,400,000
Acquisition, gain (loss) recognized										$ 0
2019 Acquisitions | HVP III | Leases, Acquired-in-Place
Self-storage facilities
Estimated life										12 months
Amortization expense												6,000,000.0	$ 3,600,000									$ 4,800,000
2019 Acquisitions | Texas
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | Florida
Self-storage facilities
Number of self-storage facilities acquired | property																											5
2019 Acquisitions | Massachusetts | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										7
2019 Acquisitions | Maryland
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | California
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | Arizona
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | Georgia
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | Georgia | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										1
2019 Acquisitions | North Carolina | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										1
2019 Acquisitions | South Carolina
Self-storage facilities
Number of self-storage facilities acquired | property																											1
2019 Acquisitions | South Carolina | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										7
2019 Acquisitions | Tennessee | HVP III
Self-storage facilities
Number of self-storage facilities acquired | property										2
2019 Acquisitions | HVP III
Self-storage facilities
Number of assets sold | property											50
2019 Acquisitions | Self-storage facilities located in US
Self-storage facilities
Aggregate purchase price																									118,300,000
2019 Acquisitions | Maryland Asset March 2019
Self-storage facilities
Number of self-storage facilities acquired | facility																										1
Aggregate purchase price																									22,000,000
2019 Dispositions
Self-storage facilities
Aggregate sale price																									$ 4,146,000
2019 Dispositions | HVP III | Leases, Acquired-in-Place
Self-storage facilities
Intangible value of the in-place leases										$ 14,300,000
2019 Dispositions | Texas
Self-storage facilities
Aggregate sale price				$ 4,100,000
Gain from sale of real estate, net				$ 1,500,000
Real Estate Acquisitions Expected in 2019 | Self storage under construction
Self-storage facilities
Development costs												$ 71,800,000						$ 71,800,000	71,800,000	$ 71,800,000	$ 71,800,000
Expected construction cost																			$ 132,400,000
Real Estate Acquisitions Expected in 2019 | Self storage under construction | Massachusetts
Self-storage facilities
Number of facilities under contract | facility																				1
Joint Venture | Self-storage Facility in Waltham, MA
Self-storage facilities
Ownership interest held by the entity (as a percent)							90.00%
Minority ownership interest (as a percent)						0.00%
Repayments of debt						$ 10,500,000
Joint Venture | Self Storage Facility in Queens, NY I
Self-storage facilities
Minority ownership interest (as a percent)								0.00%
Joint Venture | Self Storage Facility in Bayonne, NJ
Self-storage facilities
Ownership interest held by the entity (as a percent)																51.00%
Number of joint ventures | item																3
Put option ownership percentage in joint venture exercised (as a percent)					49.00%				49.00%
Option to put ownership interest in the venture					$ 15,200,000				$ 11,500,000
Joint Venture | Self Storage Facility in Brooklyn, NY I
Self-storage facilities
Put option ownership percentage in joint venture exercised (as a percent)	49.00%
Option to put ownership interest in the venture	$ 10,000,000.0
Cash portion of put option paid	$ 1,000,000.0
Units issued (in shares) | shares	276,497
Units issued	$ 9,000,000.0
HVP III
Self-storage facilities
Number of self-storage facilities acquired | property											50
Aggregate sale price											$ 293,500,000
Gain from sale of real estate, net											$ 106,700,000
Ownership interest held by the entity (as a percent)											90.00%
HVP III | Florida
Self-storage facilities
Number of assets sold | property											(3)
HVP III | Georgia
Self-storage facilities
Number of assets sold | property											(4)
HVP III | North Carolina
Self-storage facilities
Number of assets sold | property											(3)
HVP III | South Carolina
Self-storage facilities
Number of assets sold | property											(15)
HVP III | Tennessee
Self-storage facilities
Number of assets sold | property											(8)