Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Thousands						1 Months Ended	12 Months Ended
	Feb. 19, 2021 USD ($) Property	Jan. 28, 2021 USD ($) Property	Jan. 27, 2021 USD ($)	Jan. 15, 2021	Jan. 04, 2021 $ / shares	Jan. 31, 2021 USD ($) Property $ / shares shares	Dec. 31, 2020 USD ($) Property $ / shares shares	Dec. 31, 2019 USD ($) Property shares	Dec. 31, 2018 shares
Subsequent Event [Line Items]
Number of storage facilities acquired | Property							40	30
Aggregate purchase price							$ 532,625	$ 429,448
Equity Program [Member]
Subsequent Event [Line Items]
Net proceeds from issuance of common stock, shares | shares							4,091,666	0	0
Weighted average issue price | $ / shares							$ 73.16
Proceeds from issuance of common stock							$ 296,000
Self-storage Facilities [Member]
Subsequent Event [Line Items]
Number of storage facilities acquired | Property							10
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividend declared, date					Jan. 04, 2021
Dividend per common share | $ / shares					$ 0.74
Dividend paid, date					Jan. 27, 2021
Dividend record, date					Jan. 15, 2021
Dividend paid			$ 55,800
Description of stock split of common stock					three-for-two
Stock split, conversion ratio					1.5
Stock dividend payable percentage of common stock					50.00%
Description of additional share received				Shareholders of record at the close of business on January 15, 2021 received one additional share of Company stock for every two shares owned.
Subsequent Event [Member] | Equity Program [Member]
Subsequent Event [Line Items]
Net proceeds from issuance of common stock, shares | shares						1,205,009
Weighted average issue price | $ / shares						$ 78.79
Proceeds from issuance of common stock						$ 94,000
Subsequent Event [Member] | Self-storage Facilities [Member]
Subsequent Event [Line Items]
Number of storage facilities acquired | Property	1					2
Aggregate purchase price	$ 48,600					$ 26,300
Subsequent Event [Member] | Self-storage Facilities [Member] | Acquisition Contract [Member]
Subsequent Event [Line Items]
Number of storage facilities acquired | Property		2
Aggregate purchase price		$ 39,300