Real Estate (Significant Acquisitions and Dispositions) (Details)								1 Months Ended	3 Months Ended			12 Months Ended
		Dec. 11, 2019 USD ($) a	Nov. 15, 2019 USD ($) ft²	Sep. 13, 2019 USD ($) ft²	Feb. 08, 2019 USD ($) ft²	Nov. 29, 2018 USD ($) ft²	Jun. 15, 2018 USD ($) a ft²	Aug. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) ft² condominiums	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) ft² condominiums	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition and Disposition
Distribution from partnerships in excess of earnings												$ 2,765,000	$ 275,000	$ 1,729,000
Our investment in joint venture									$ 28,604,000			28,604,000	26,859,000
Assets held for sale									$ 1,729,000			1,729,000	16,576,000
Mortgage loans assumed with acquisition												98,041,000	0	79,401,000
Gain on sale of real estate, net of income taxes												$ 116,393,000	11,915,000	$ 77,922,000
Fairfax Junction
Business Acquisition and Disposition
Square footage of real estate property | ft²					75,000
Purchase price of real estate acquisition	[1]				$ 22,500,000
Georgetowne Shopping Center
Business Acquisition and Disposition
Square footage of real estate property | ft²			147,000
Purchase price of real estate acquisition	[2]		$ 83,700,000
Hoboken
Business Acquisition and Disposition
Square footage of real estate property | ft²									158,000			158,000
Purchase price of real estate acquisition	[3]											$ 189,200,000
Mortgage loans assumed with acquisition												$ 98,000,000.0
Number of Units in Real Estate Property									123			123
Ownership percentage in partnership									90.00%			90.00%
Freedom Plaza
Business Acquisition and Disposition
Square footage of real estate property | ft²							113,000
Construction pre-funding investment							$ 34,400,000
Controlling interest percentage							91.00%
Land parcel acre on which the shopping center will be constructed | a							9.4
Contribution from third party bank							$ 13,900,000
Strike price of put option							1,000
Direct and incremental costs of structuring the NMTC transaction							$ 1,600,000
Cash in escrow to be used for development									$ 12,600,000			$ 12,600,000
Two Properties [Member]
Business Acquisition and Disposition
Proceeds from sale													42,200,000
Gain on sale of real estate, net													4,700,000
A building adjacent to Bell Gardens
Business Acquisition and Disposition
Square footage of real estate property | ft²						40,000
Purchase price of real estate acquisition						$ 9,600,000
San Antonio Center
Business Acquisition and Disposition
Square footage of real estate property		11.7		6,000
Estimated Litigation Liability		$ 45,500,000
Proceeds from sale		$ 154,700,000
Gain on sale									85,100,000
Purchase price of real estate acquisition				$ 6,500,000
Three properties [Member]
Business Acquisition and Disposition
Proceeds from sale												149,000,000.0
Gain on sale												28,300,000
Other Assets | Above Market Leases | Fairfax Junction
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities					600,000
Other Assets | Above Market Leases | Georgetowne Shopping Center
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities			2,000,000.0
Other Assets | Above Market Leases | Hoboken
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities												3,600,000
Other Liabilities | Fairfax Junction
Business Acquisition and Disposition
Net assets acquired allocated to below market leases					$ 400,000
Other Liabilities | Georgetowne Shopping Center
Business Acquisition and Disposition
Net assets acquired allocated to below market leases			$ 400,000
Other Liabilities | Hoboken
Business Acquisition and Disposition
Net assets acquired allocated to below market leases												8,100,000
Condominiums | Assembly Row and Pike & Rose
Business Acquisition and Disposition
Proceeds from sale												$ 20,100,000	$ 133,500,000
Number of condominium units sold												43	176
Taxes related to the sale of condominiums													$ 1,600,000
Assets held for sale									$ 1,700,000			$ 1,700,000	16,600,000
Number of Units in Real Estate Property | condominiums									221			221
Gain on sale of real estate, net of income taxes										$ 4,000,000.0	$ 3,300,000	$ 2,600,000	$ 7,200,000
Equity method investments | Hotel | Assembly Row Hotel
Business Acquisition and Disposition
Contribution of Property								$ 44,000,000.0
Distribution from partnerships in excess of earnings								$ 38,000,000.0
Our investment in joint venture									$ 3,200,000			$ 3,200,000

[1] Approximately $0.6 million and $0.4 million of net assets acquired were allocated to other assets for "above market leases," and other liabilities for "below market leases," respectively.

[2] Approximately $2.0 million and $0.4 million of net assets acquired were allocated to other assets for "above market leases," and other liabilities for "below market leases," respectively.

[3] These acquisitions were completed through a newly formed joint venture, for which we own a 90% interest. The purchase price includes new and assumptions of mortgage debt totaling approximately $98.0 million. This property includes 123 residential units in addition to the GLA in the table above. Approximately $3.6 million and $8.1 million of net assets acquired were allocated to other assets for "above market leases," and other liabilities for "below market leases," respectively.