INVESTMENT ACTIVITY (Details)											3 Months Ended					6 Months Ended					12 Months Ended					18 Months Ended
	Dec. 16, 2019 USD ($)	Oct. 07, 2019 USD ($)	Sep. 17, 2019 USD ($)	Sep. 06, 2019 USD ($)	Aug. 08, 2019 USD ($)	Jun. 28, 2019 USD ($)	Jun. 25, 2019 USD ($)	Jun. 06, 2019 USD ($) property	Jun. 05, 2019 USD ($) property	Jan. 31, 2018 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 item	Dec. 31, 2015 item	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) facility	Jun. 30, 2020 USD ($) property	Jun. 30, 2019 USD ($)	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)																										51.00%
Total Construction Costs																					$ 136,500,000					$ 45,900,000
Acquisition of noncontrolling interest												$ 27,618,000	$ 10,000,000
HVP III
Self-storage facilities
Percentage of interest acquired								90.00%
Texas
Self-storage facilities
Cash paid for acquisition										$ 200,000
Issuance of OP Shares										4,800,000
Total consideration										$ 12,200,000
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
Number of facilities under contract | property																			4
Self storage under construction | New York
Self-storage facilities
Number of facilities under contract | facility																		1
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
Cash paid for acquisition				$ 2,600,000
Percentage of interest acquired				10.00%
Number of facilities, construction completed and opened																										$ 1
Ownership interest held by the entity (as a percent)					90.00%																					100.00%
Total Construction Costs																										$ 18,000,000
Acquisition of noncontrolling interest					$ 2,000,000.0
Repayments of debt					$ 10,500,000
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 Waltham, MA and New York, NY
Self-storage facilities
Acquisition, gain (loss) recognized				$ 0
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											$ 250,000,000					$ 250,000,000	250,000,000	$ 250,000,000	$ 250,000,000							250,000,000
Estimated life																12 months
Amortization expense											400,000						500,000
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											1,200,000	$ 400,000					2,700,000			$ 400,000
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											2,400,000						6,000,000.0
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											$ 51,400,000					$ 51,400,000	51,400,000	$ 51,400,000	$ 51,400,000							$ 51,400,000
Expected construction cost																	$ 94,600,000
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
Minority ownership interest (as a percent)					0.00%
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														2
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
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)