Real Estate (Significant Acquisitions and Dispositions) (Details)														1 Months Ended	3 Months Ended								12 Months Ended
	Nov. 29, 2018 USD ($) ft²	Nov. 09, 2018 USD ($)	Aug. 16, 2018 USD ($)	Jun. 15, 2018 USD ($) a ft²	Dec. 28, 2017 USD ($)	Sep. 25, 2017 USD ($)	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 ($) ft²	Aug. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) project	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 28, 2017 USD ($)	Dec. 31, 2018 USD ($) project	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition and Disposition
Distribution from real estate partnership in excess of earnings																							$ 275,000	$ 1,729,000	$ 3,910,000
Our investment in joint venture															$ 26,859,000				$ 23,941,000				26,859,000	23,941,000
Assets held for sale															$ 16,576,000				0				$ 16,576,000	0
Number of real estate properties | project															104								104
Mortgage loans assumed with acquisition																							$ 0	79,401,000	34,385,000
Jordan Downs 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															$ 32,200,000								32,200,000
Atlantic Plaza
Business Acquisition and Disposition
Proceeds from sale		$ 27,200,000
Gain on sale		$ 1,600,000													1,600,000
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
Hastings Ranch Plaza
Business Acquisition and Disposition
Square footage of real estate property | ft²													274,000
Purchase price of real estate acquisition													$ 29,500,000
Riverpoint Center
Business Acquisition and Disposition
Square footage of real estate property | ft²												211,000
Purchase price of real estate acquisition												$ 107,000,000
Fourth Street
Business Acquisition and Disposition
Square footage of real estate property | ft²										71,000
Controlling interest percentage										90.00%
Gross value of acquisition										$ 23,900,000
Noncontrolling interest allocated in acquisition										2,400,000
Primestor
Business Acquisition and Disposition
Square footage of real estate property | ft²								1,352,000
Gross value of acquisition								$ 357,000,000
Noncontrolling interest allocated in acquisition								$ 30,600,000
Azalea
Business Acquisition and Disposition
Square footage of real estate property | ft²								222,000
Bell Gardens
Business Acquisition and Disposition
Square footage of real estate property | ft²								330,000
La Alameda
Business Acquisition and Disposition
Square footage of real estate property | ft²								245,000
Ownership percentage in partnership								24.50%
Olivo at Mission Hills
Business Acquisition and Disposition
Square footage of real estate property | ft²								155,000
Plaza Del Sol
Business Acquisition and Disposition
Square footage of real estate property | ft²								48,000
Plaza Pacoima
Business Acquisition and Disposition
Square footage of real estate property | ft²								204,000
Sylmar Towne Center
Business Acquisition and Disposition
Square footage of real estate property | ft²								148,000
150 Post Street
Business Acquisition and Disposition
Proceeds from sale							$ 69,300,000
Gain on sale							$ 45,200,000
North Lake Commons
Business Acquisition and Disposition
Proceeds from sale						$ 15,600,000
Gain on sale						$ 4,900,000
Primestor
Business Acquisition and Disposition
Number of real estate properties | property								6
Asset Acquisitions, Pro Forma depreciation and amortization																								11,500,000	11,400,000
Asset Acquisitions Pro Forma Revenue																								872,900,000	826,600,000
Asset Acquisitions, Pro Forma Net Inome (Loss)																								284,600,000	$ 244,300,000
Other Assets | Intangible lease assets acquired | Hastings Ranch Plaza
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities													21,500,000
Other Assets | Intangible lease assets acquired | Primestor
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities								$ 7,800,000
Other Assets | Above Market Leases | Riverpoint Center
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities												1,000,000
Other Assets | Above Market Leases | Fourth Street
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities										800,000
Other Liabilities | Riverpoint Center
Business Acquisition and Disposition
Net assets acquired allocated to below market leases												$ 12,300,000
Other Liabilities | Fourth Street
Business Acquisition and Disposition
Net assets acquired allocated to below market leases										$ 300,000
Other Liabilities | Intangible lease liabilities acquired | Hastings Ranch Plaza
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities													$ 15,200,000
Other Liabilities | Intangible lease liabilities acquired | Primestor
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities								$ 36,200,000
FRIT/Primestor JV
Business Acquisition and Disposition
Controlling interest percentage								90.00%
FRIT/Primestor JV | La Alameda
Business Acquisition and Disposition
Purchase price of equity method investment								$ 19,800,000
Mortgage loan on unconsolidated real estate property								41,000,000
Our share of unconsolidated mortgage debt								10,000,000
Residential | Chelsea
Business Acquisition and Disposition
Proceeds from sale			$ 15,000,000
Gain on sale			$ 3,100,000													$ 3,100,000
Land | Assembly Row
Business Acquisition and Disposition
Proceeds from sale									$ 17,300,000		$ 36,000,000
Gain on sale																						$ 15,400,000
Number of members with purchase options											2
Land | A parcel of land at Bethesda Row
Business Acquisition and Disposition
Proceeds from sale					$ 8,500,000
Gain on sale					$ 6,500,000
Condominiums | Assembly Row and Pike & Rose
Business Acquisition and Disposition
Proceeds from sale																							133,500,000
Gain on sale																	$ 4,000,000	$ 3,300,000	1,500,000	$ 600,000	$ 3,300,000		$ 7,200,000
Number of condominium units sold																							176
Taxes related to the sale of condominiums																							$ 1,600,000
Assets held for sale															16,600,000								16,600,000
Condominiums | Assembly Row
Business Acquisition and Disposition
Taxes related to the sale of condominiums																								1,400,000
Gain on sale of real estate, net of income taxes																								5,400,000
Accounts receivable																			$ 67,100,000					$ 67,100,000
Primestor | Mortgages
Business Acquisition and Disposition
Mortgage loans assumed with acquisition								$ 79,400,000
Equity method investments | Hotel | Assembly Row Hotel
Business Acquisition and Disposition
Contribution of Property														$ 44,000,000
Distribution from real estate partnership in excess of earnings														$ 38,000,000
Our investment in joint venture															$ 5,600,000								$ 5,600,000