Dispositions (Narratives) (Details) ft² in Thousands							3 Months Ended				9 Months Ended
	Mar. 31, 2018 USD ($)	Sep. 09, 2015 USD ($) ft²	Aug. 06, 2015 USD ($)	Mar. 31, 2015 USD ($) ft²	Mar. 13, 2015 USD ($)	Jan. 15, 2015 USD ($) Properties	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($) Properties	Sep. 30, 2014 USD ($)
Dispositions [Abstract]
Number Of Real Estate Properties Sold | Properties											5
Cash Considerations Transferred During Spin Off (in US Dollars)											$ 225,000,000	$ 0
Gain on sale of interest in leases							$ 0		$ 0		21,376,000	0
Cash Proceeds from sale of real estate											10,731,000
Net gains on sale of real estate							$ 0	$ 3,675,000	57,796,000		10,867,000	57,796,000
Impairment Losses											$ 256,000	20,842,000
Pennsylavenia Real Estate Investment Trust [Member]
Dispositions [Abstract]
Equity method ownership percentage				8.10%			8.10%	8.10%			8.10%
Deferred Gain on Sale of Property				$ 631,000				$ 631,000
Retail Springfield Mall [Member] | Retail Segment Regional Malls [Member]
Dispositions [Abstract]
Net gains on sale of real estate				$ 7,823,000				$ 7,192,000
Impairment Losses										$ 20,000,000
Retail Springfield Mall [Member] | Retail Segment Regional Malls [Member] | Pennsylavenia Real Estate Investment Trust [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²				1,350				1,350
Gross proceeds from the sale of real estate				$ 485,313,000
Cash Proceeds from sale of real estate				$ 340,000,000
Retail Springfield Mall [Member] | Retail Segment Regional Malls [Member] | Scenario Forecast [Member]
Dispositions [Abstract]
Additional Consideration Receivable Percent	50.00%
Additional Consideration Theshold, Value	$ 465,000,000
Capitalization rate	5.50%
Geary Street, CA [Member]
Dispositions [Abstract]
Proceeds from Sale of interest in leases					$ 34,189,000
Gain on sale of interest in leases					$ 21,376,000
Monmouth Mall [Member]
Dispositions [Abstract]
Net gains on sale of real estate							$ 33,153,000		$ 0		$ 33,153,000	$ 0
Monmouth Mall [Member] | Retail Segment Regional Malls [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate			$ 38,000,000
Market value of the property sold			229,000,000
Net gains on sale of real estate			$ 33,153,000
Sale Of Ownership Interest Percent			50.00%
Urban Edge Properties [Member]
Dispositions [Abstract]
Cash Considerations Transferred During Spin Off (in US Dollars)						$ 225,000,000
Equity method ownership percentage						5.40%
Urban Edge Properties [Member] | Retail Segment Strip Shopping Centers [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties						79
Urban Edge Properties [Member] | Retail Segment Regional Malls [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties						3
Urban Edge Properties [Member] | Warehouse [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties						1
Washington DC Segment [Member] | 1750 Pennsylvania Avenue [Member] | Office Building [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²		278
Gross proceeds from the sale of real estate		$ 182,000,000
Net gains on sale of real estate		102,000,000
Deferred Tax Gain On Sale Of Real Estate		$ 137,000,000