Investments in and Advances to Joint Ventures - Summary of Preferred investments (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) ShoppingCenter	Dec. 31, 2018 USD ($)	Dec. 31, 2015 USD ($) ShoppingCenter	Oct. 31, 2014 USD ($) ShoppingCenter
Schedule Of Equity Method Investments [Line Items]
Preferred Investment, Valuation Allowance	$ (84.5)	$ (72.4)
BRE DDR Retail Holdings Joint Venture Acquisitions [Member] | BRE DDR III [Member]
Schedule Of Equity Method Investments [Line Items]
Unconsolidated Joint Venture Partner Formation Date Month And Year	2014
Preferred Investment (Principal)	$ 178.9			$ 300.0
Preferred Investment, Valuation Allowance	(73.2)
Preferred Investment, Net of Reserve	$ 105.7
Properties Owned | ShoppingCenter	14			70
BRE DDR Retail Holdings Joint Venture Acquisitions [Member] | BRE DDR IV [Member]
Schedule Of Equity Method Investments [Line Items]
Unconsolidated Joint Venture Partner Formation Date Month And Year	2015
Preferred Investment (Principal)	$ 64.1		$ 82.6
Preferred Investment, Valuation Allowance	(11.3)
Preferred Investment, Net of Reserve	$ 52.8
Properties Owned | ShoppingCenter	5		6
BRE DDR Retail Holdings Joint Venture Acquisitions [Member] | BRE DDR III and IV [Member]
Schedule Of Equity Method Investments [Line Items]
Preferred Investment (Principal)	$ 243.0		$ 382.6
Preferred Investment, Valuation Allowance	(84.5)
Preferred Investment, Net of Reserve	$ 158.5