SUBSEQUENT EVENTS (Details)					1 Months Ended
	Oct. 30, 2020 USD ($)	Oct. 21, 2020 USD ($) property item	Oct. 06, 2020 USD ($)	Sep. 29, 2020 USD ($)	Oct. 31, 2020 USD ($) property	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Senior notes 4.800% due 2022
Subsequent Events
Senior notes, principal amount						$ 250,000,000	$ 250,000,000
Interest rate (as a percent)						4.80%
Effective interest rate (as a percent)						4.82%
New York
Subsequent Events
Total consideration				$ 10,000,000.0
Cash paid for acquisition				$ 1,000,000.0
Subsequent event | Senior notes 4.800% due 2022
Subsequent Events
Repayment of outstanding debt	$ 250,000,000.0
Interest rate (as a percent)	4.80%
Redemption price	$ 1,084.34
Redemption price, denominator	$ 1,000
Subsequent event | Senior Notes 2.000 % Due 2031
Subsequent Events
Senior notes, principal amount			$ 450,000,000.0
Interest rate (as a percent)			2.00%
Pricing percentage			99.074%
Net Proceeds			$ 442,100,000
Effective interest rate (as a percent)			2.10%
Subsequent event | New York
Subsequent Events
Number of self-storage facilities acquired | property		8
Total consideration		$ 540,000,000.0
Cash paid for acquisition		201,700,000
Issuance of OP Shares		183,700,000
Assumed debt		$ 154,600,000
Number of tranches | item		2
Subsequent event | Self-storage facilities
Subsequent Events
Number of self-storage facilities acquired | property					7
Total consideration					$ 89,200,000