Investments in and Advances to Joint Ventures - Summary of Preferred investments (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) ShoppingCenter	Dec. 31, 2019 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)
Schedule Of Equity Method Investments [Line Items]
Preferred Investment, Valuation Allowance	$ (107.4)	$ (87.9)
BRE DDR III [Member]
Schedule Of Equity Method Investments [Line Items]
Redemption Date	2021-10
Preferred Investment (Principal)	$ 132.4			$ 300.0
Preferred Investment, Valuation Allowance	(95.8)
Preferred Investment, Net of Reserve	$ 36.6
Properties Owned | ShoppingCenter	13
BRE DDR IV
Schedule Of Equity Method Investments [Line Items]
Redemption Date	2022-12
Preferred Investment (Principal)	$ 64.1		$ 82.6
Preferred Investment, Valuation Allowance	(11.6)
Preferred Investment, Net of Reserve	$ 52.5
Properties Owned | ShoppingCenter	5
BRE DDR III and IV [Member]
Schedule Of Equity Method Investments [Line Items]
Preferred Investment (Principal)	$ 196.5		$ 382.6
Preferred Investment, Valuation Allowance	(107.4)
Preferred Investment, Net of Reserve	$ 89.1