Shareholders' Equity - Additional Information (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) Property $ / shares shares	Sep. 30, 2019 shares	Dec. 31, 2019 shares	Jun. 14, 2018 USD ($)	Aug. 02, 2017 USD ($)
Stockholders Equity [Line Items]
Common stock value authorized under equity offering program				$ 300,000,000
Number of storage facilities acquired | Property	31
Buyback Program, authorized repurchase amount					$ 200,000,000
Stock repurchased during period, shares | shares	0		0
Shares issued under Dividend Reinvestment Plan | shares	0	0
Equity Program [Member]
Stockholders Equity [Line Items]
Common stock shares issued under equity offering program | shares	1,475,949	0
Weighted average issue price | $ / shares	$ 106.64
Proceeds from issuance of common stock	$ 155,800,000
Sales commissions paid	1,600,000
Other expenses	$ 200,000