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Real Estate (Significant Acquisitions and Dispositions) (Details) $ in Thousands									3 Months Ended			9 Months Ended
	Sep. 25, 2017 USD ($)	Aug. 25, 2017 USD ($)	Aug. 02, 2017 USD ($) ft² property	Jun. 28, 2017 USD ($)	May 19, 2017 USD ($) ft²	Apr. 04, 2017 USD ($)	Mar. 31, 2017 USD ($) ft²	Feb. 01, 2017 USD ($) ft²	Sep. 30, 2017 USD ($) project	Jun. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) project	Sep. 30, 2016 USD ($)
Business Acquisitions and Dispositions
Number of real estate properties \| project									104			104
Mortgage loans assumed with acquisition												$ 79,401	$ 34,385
FRIT/Primestor JV
Business Acquisitions and Dispositions
Ownership interest			90.00%
Hastings Ranch Plaza
Business Acquisitions and Dispositions
Square footage \| ft²								274,000
Purchase price								$ 29,500
Hastings Ranch Plaza \| Other Assets \| Lease intangibles
Business Acquisitions and Dispositions
Net assets acquired								21,500
Hastings Ranch Plaza \| Other Liabilities \| Lease liabilities
Business Acquisitions and Dispositions
Net assets acquired								$ 15,200
Riverpoint Center
Business Acquisitions and Dispositions
Square footage \| ft²							211,000
Purchase price							$ 107,000
Riverpoint Center \| Other Assets \| Above market leases
Business Acquisitions and Dispositions
Net assets acquired							1,000
Riverpoint Center \| Other Liabilities
Business Acquisitions and Dispositions
Net assets acquired allocated to below market leases							$ 12,300
Assembly Row \| Land
Business Acquisitions and Dispositions
Number of ground lessees						2
Proceeds from sale				$ 17,300		$ 36,000
Gain on sale of real estate, net										$ 15,400
Assembly Row \| Condominiums
Business Acquisitions and Dispositions
Gain on sale of real estate, net									$ 600			3,900
Taxes related to the sale of condominiums									300			2,000
Receivable recognized on the sale of condominiums									59,100			59,100
Mixed-use property located in Berkeley, California
Business Acquisitions and Dispositions
Square footage \| ft²					71,000
Gross value					$ 23,900
Ownership interest					90.00%
Contributions from noncontrolling interests					$ 2,400
Mixed-use property located in Berkeley, California \| Other Assets \| Above market leases
Business Acquisitions and Dispositions
Net assets acquired					800
Mixed-use property located in Berkeley, California \| Other Liabilities
Business Acquisitions and Dispositions
Net assets acquired allocated to below market leases					$ 300
Primestor
Business Acquisitions and Dispositions
Square footage \| ft²			1,352,000
Gross value			$ 357,000
Contributions from noncontrolling interests			30,600
Primestor \| Other Assets \| Lease intangibles
Business Acquisitions and Dispositions
Net assets acquired			7,800
Primestor \| Other Liabilities \| Lease liabilities
Business Acquisitions and Dispositions
Net assets acquired			$ 36,200
La Alameda
Business Acquisitions and Dispositions
Square footage \| ft²			245,000
Equity Method Investments			24.50%
La Alameda \| FRIT/Primestor JV
Business Acquisitions and Dispositions
Purchase price of equity method investment			$ 19,800
Mortgage Debt			41,000
Our share of mortgage debt			$ 10,000
Azalea
Business Acquisitions and Dispositions
Square footage \| ft²			222,000
Bell Gardens
Business Acquisitions and Dispositions
Square footage \| ft²			330,000
Olivo at Mission Hills
Business Acquisitions and Dispositions
Square footage \| ft²			155,000
Plaza Del Sol
Business Acquisitions and Dispositions
Square footage \| ft²			48,000
Plaza Pacoima
Business Acquisitions and Dispositions
Square footage \| ft²			204,000
Sylmar Towne Center
Business Acquisitions and Dispositions
Square footage \| ft²			148,000
150 Post Street
Business Acquisitions and Dispositions
Proceeds from sale		$ 69,300
Gain (Loss) on Sale of Properties		$ 45,200
North Lake Commons
Business Acquisitions and Dispositions
Proceeds from sale	$ 15,600
Gain (Loss) on Sale of Properties	$ 4,900
Primestor
Business Acquisitions and Dispositions
Number of real estate properties \| property			6
Asset Acquisitions, Pro Forma depreciation and amortization									2,800		$ 2,800	8,500	8,500
Asset Acquisitions Pro Forma Revenue									220,200		207,300	649,000	616,000
Asset Acquisitions, Pro Forma Net Inome (Loss)									$ 106,200		$ 57,500	$ 235,900	$ 187,500
Primestor \| Mortgage loan
Business Acquisitions and Dispositions
Mortgage loans assumed with acquisition			$ 79,400