Mortgages Payable and Debt Maturities - Summary of Mortgage Payable (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Facility
4.98% Mortgage Note Due January 1, 2021 [Member]
Debt Instrument [Line Items]
Interest rate	4.98%
Mortgage note due date	Jan. 01, 2021
Self-storage facilities | Facility	1
Aggregate net book value of property pledged for mortgage note | $	$ 9.2
Effective interest rate	5.30%
4.065% Mortgage Note Due April 1, 2023 [Member]
Debt Instrument [Line Items]
Interest rate	4.065%
Mortgage note due date	Apr. 01, 2023
Self-storage facilities | Facility	1
Aggregate net book value of property pledged for mortgage note | $	$ 7.2
Effective interest rate	4.32%
5.26% Mortgage Note Due November 1, 2023 [Member]
Debt Instrument [Line Items]
Interest rate	5.26%
Mortgage note due date	Nov. 01, 2023
Self-storage facilities | Facility	1
Aggregate net book value of property pledged for mortgage note | $	$ 7.8
Effective interest rate	5.59%
4.4625% Mortgage Note Due December 6, 2024 [Member]
Debt Instrument [Line Items]
Interest rate	4.4625%
Mortgage note due date	Dec. 06, 2024
Self-storage facilities | Facility	3
Aggregate net book value of property pledged for mortgage note | $	$ 54.9
Effective interest rate	3.18%
5.99% Mortgage Note Due May 1, 2026 [Member]
Debt Instrument [Line Items]
Interest rate	5.99%
Mortgage note due date	May 01, 2026
Self-storage facilities | Facility	1
Aggregate net book value of property pledged for mortgage note | $	$ 6.4
Effective interest rate	6.32%
4.44% Mortgage Note Due July 6, 2025 [Member]
Debt Instrument [Line Items]
Interest rate	4.44%
Mortgage note due date	Jul. 06, 2025
Self-storage facilities | Facility	1
Aggregate net book value of property pledged for mortgage note | $	$ 13.6
Effective interest rate	4.42%