Real Estate (Significant Acquisitions and Dispositions) (Details) $ in Thousands														3 Months Ended					12 Months Ended			24 Months Ended
	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 ($) ft²	Nov. 07, 2016 USD ($)	Jul. 26, 2016 USD ($)	May 12, 2016 USD ($)	Jan. 13, 2016 USD ($) property	Dec. 31, 2017 USD ($) project	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 28, 2017 USD ($)	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2017 USD ($) project	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition and Disposition
Number of real estate properties | project														104					104
Mortgage loans assumed with acquisition																			$ 79,401	$ 34,385	$ 89,516
Contributions from noncontrolling interests																			35,331	885	$ 34,737
Assets held for sale														$ 0					0	33,856		$ 33,856
Hastings Ranch Plaza
Business Acquisition and Disposition
Square footage of real estate property | ft²									274,000
Purchase price of real estate acquisition									$ 29,500
Riverpoint Center
Business Acquisition and Disposition
Square footage of real estate property | ft²								211,000										211,000
Purchase price of real estate acquisition								$ 107,000
Fourth Street
Business Acquisition and Disposition
Square footage of real estate property | ft²						71,000
Gross value of acquisition						$ 23,900
Controlling interest percentage						90.00%
Noncontrolling interest allocated in acquisition						$ 2,400
Primestor
Business Acquisition and Disposition
Square footage of real estate property | ft²				1,352,000
Gross value of acquisition				$ 357,000
Noncontrolling interest allocated in acquisition				$ 30,600
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
Gain (Loss) on Sale of Properties			$ 45,200
North Lake Commons
Business Acquisition and Disposition
Proceeds from sale		$ 15,600
Gain (Loss) on Sale of Properties		$ 4,900
Primestor
Business Acquisition and Disposition
Number of real estate properties | property				6
Asset Acquisitions, Pro Forma depreciation and amortization																			11,500	11,400
Asset Acquisitions Pro Forma Revenue																			872,900	826,600
Asset Acquisitions, Pro Forma Net Inome (Loss)																			284,600	244,300
Clarion
Business Acquisition and Disposition
Purchase Price													$ 153,700
Ownership percentage in partnership													70.00%
Number of real estate properties | property													6
Cash paid in acquisition													$ 130,000
Mortgage loans assumed with acquisition													34,400
Gain on acquisition of controlling interest													25,700					$ 25,700
A building adjacent to our Barcroft Plaza property
Business Acquisition and Disposition
Purchase Price										$ 5,300
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
Other Assets | Intangible lease assets acquired | Primestor
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities				$ 7,800
Other Assets | Above Market Leases | Riverpoint Center
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities								1,000
Other Assets | Above Market Leases | Fourth Street
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities						800
Other Assets | Above Market Leases | Clarion
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities													7,300
Other Liabilities | Riverpoint Center
Business Acquisition and Disposition
Net assets acquired allocated to below market leases								$ 12,300
Other Liabilities | Fourth Street
Business Acquisition and Disposition
Net assets acquired allocated to below market leases						$ 300
Other Liabilities | Clarion
Business Acquisition and Disposition
Net assets acquired allocated to below market leases													$ 4,900
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
Other Liabilities | Intangible lease liabilities acquired | Primestor
Business Acquisition and Disposition
Net assets acquired allocated to intangible lease assets or liabilities				$ 36,200
General and Administrative Expense | Clarion
Business Acquisition and Disposition
Acquisition costs																				100		200
General and Administrative Expense | A building adjacent to our Barcroft Plaza property
Business Acquisition and Disposition
Acquisition costs																				100
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
Mortgage Loan on Unconsolidated Real Estate Property				41,000
Our share of unconsolidated mortgage debt				10,000
CocoWalk | Coconut Grove
Business Acquisition and Disposition
Gain on sale of real estate, net of noncontrolling interests												$ 500
CocoWalk | Coconut Grove
Business Acquisition and Disposition
Purchase Price											$ 5,900
CocoWalk | General and Administrative Expense | Coconut Grove
Business Acquisition and Disposition
Acquisition costs																				200
Land | Assembly Row
Business Acquisition and Disposition
Number of members with purchase options							2
Proceeds from sale					$ 17,300		$ 36,000
Gain (Loss) on Sale of Properties																	$ 15,400
Gain on sale of real estate, net of income taxes																$ 15,400
Assets held for sale																				$ 33,900		$ 33,900
Land | Bethesda Row
Business Acquisition and Disposition
Proceeds from sale	$ 8,500
Gain (Loss) on Sale of Properties	$ 6,500
Condominiums | Assembly Row
Business Acquisition and Disposition
Gain on sale of real estate, net of income taxes														1,500	$ 600	$ 3,300			5,400
Income taxes related to gain on sale																			1,400
Accounts Receivable, Gross														$ 67,100					$ 67,100
Primestor | Mortgages
Business Acquisition and Disposition
Mortgage loans assumed with acquisition				$ 79,400