Dispositions (Narratives) (Details) $ / shares in Units, ft² in Thousands																3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 22, 2015 USD ($) ft²	Sep. 09, 2015 USD ($) ft²	Aug. 06, 2015 USD ($)	Mar. 31, 2015 USD ($) ft² $ / shares shares	Mar. 13, 2015 USD ($)	Jan. 15, 2015 USD ($) Properties shares	Dec. 18, 2014 USD ($) ft²	Jul. 08, 2014 USD ($) ft²	Mar. 02, 2014 USD ($) ft² $ / shares shares	Feb. 24, 2014 USD ($)	Dec. 17, 2013 USD ($) ft²	Sep. 23, 2013 USD ($)	Apr. 15, 2013 USD ($)	Jan. 24, 2013 USD ($)	Mar. 31, 2015 USD ($) ft² $ / shares	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) Properties	Dec. 31, 2014 USD ($) Properties	Dec. 31, 2013 USD ($) Properties
Dispositions [Abstract]
Income from discontinued operations																		$ 52,262,000	$ 585,676,000	$ 568,095,000
Net gains on sale of real estate																		65,396,000	507,192,000	414,502,000
Cash Considerations Transferred During Spin Off (in US Dollars)																		225,000,000	0	0
Gain on sale of interest in leases																		0	0	1,377,000
Operating Partnership Units Value																		80,000,000	0	0
Impairment Losses																		$ 256,000	26,518,000	37,170,000
Pennsylvania Real Estate Investment Trust [Member]
Dispositions [Abstract]
Equity method ownership percentage					8.10%											8.10%		8.10%
Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Net gains on sale of real estate																			$ 22,500,000
Number Of Real Estate Properties | Properties																			22
Number Of Real Estate Properties Sold | Properties																			6
Cash Proceeds from sale of real estate																			$ 66,410,000
Springfield Mall [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Net gains on sale of real estate					$ 7,823,000											$ 7,192,000
Deferred Gain on Sale of Property					$ 631,000											$ 631,000
Impairment Losses																	$ 20,000,000
Springfield Mall [Member] | Pennsylvania Real Estate Investment Trust [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²					1,350					1,350						1,350
Gross proceeds from the sale of real estate					$ 485,313,000					$ 485,313,000
Cash Proceeds from sale of real estate					$ 340,000,000					$ 340,000,000
Operating Partnership Units Received From Buyer | shares					6,250,000					6,250,000
Operating Partnership Units Value					$ 145,313,000					$ 145,313,000						$ 145,313,000
Operating Partnership Unis Value Per Unit | $ / shares					$ 23.25					$ 23.25						$ 23.25
Impairment Losses																	$ 20,000,000
Springfield Mall [Member] | Scenario Forecast [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Additional Consideration Receivable Percent	50.00%
Additional Consideration Theshold, Value	$ 465,000,000
Capitalization rate	5.50%
Geary Street, CA [Member] | Discontinued Operations [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] | Regional Malls [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate				$ 38,000,000
Net gains on sale of real estate				33,153,000
Market value of the property sold				$ 229,000,000
Sale Of Ownership Interest Percent				50.00%
Residual Retail Properties [Member]
Dispositions [Abstract]
Net gains on sale of real estate																		$ 3,675,000
Number Of Real Estate Properties Sold | Properties																		5
Cash Proceeds from sale of real estate																		$ 10,731,000
Urban Edge Properties [Member]
Dispositions [Abstract]
Operating Partnership Units Received From Buyer | shares							5,717,184
Urban Edge Properties [Member] | Discontinued Operations [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] | Warehouse [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties							1
Urban Edge Properties [Member] | Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties							79
Urban Edge Properties [Member] | Regional Malls [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Number Of Real Estate Properties | Properties							3
Retail Properties Segment [Member] | Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Net gains on sale of real estate																				$ 7,851,000
Number Of Real Estate Properties Sold | Properties																				12
Cash Proceeds from sale of real estate																				$ 82,300,000
Retail Properties Segment [Member] | Retail NY Hicksville (Broadway Mall) [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate											$ 94,000,000
Net proceeds from the sale of real estate											$ 92,174,000
Retail Properties Segment [Member] | Retail CA Los Angeles Beverly Connection [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²									335
Gross proceeds from the sale of real estate									$ 260,000,000
Net gains on sale of real estate									44,155,000
Cash Proceeds from sale of real estate									239,000,000
Mortgage loan receivable									$ 21,000,000
Mortgage loan receivable maturity tenure									10 years
Retail Properties Segment [Member] | Green Acres Mall [Member] | Regional Malls [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate															$ 500,000,000
Net gains on sale of real estate															202,275,000
Net proceeds from the sale of real estate															$ 185,000,000
Retail Properties Segment [Member] | Plant [Member] | Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate														$ 203,000,000
Net gains on sale of real estate														32,169,000
Net proceeds from the sale of real estate														98,000,000
Retail Properties Segment [Member] | Gallery At Market Street [Member] | Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate														60,000,000
Net gains on sale of real estate														33,058,000
Net proceeds from the sale of real estate														$ 58,000,000
Retail Properties Segment [Member] | Tampa Florida [Member] | Strip Shopping Centers [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate													$ 45,000,000
Retail Properties Segment [Member] | Tampa Florida [Member] | Strip Shopping Centers [Member] | Vornado Realty Trust [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate													33,750,000
Net gains on sale of real estate													8,728,000
Net proceeds from the sale of real estate													$ 20,810,000
Sale Of Ownership Interest Percent													75.00%
Washington DC Segment [Member] | 1750 Pennsylvania Avenue [Member] | Office Building [Member] | Discontinued Operations [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
Other Segment [Member]
Dispositions [Abstract]
Income from discontinued operations																		$ 52,262,000	$ 122,513,000	$ 407,781,000
New York Segment [Member] | 20 Broad Street [Member] | Office Building [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²		473
Gross proceeds from the sale of real estate		$ 200,000,000
Income from discontinued operations		157,000,000
Net gains on sale of real estate		142,000,000
Lease termination income		$ 15,000,000
New York Segment [Member] | 1740 Broadway [Member] | Office Building [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²								601
Gross proceeds from the sale of real estate								$ 605,000,000
Net gains on sale of real estate								441,000,000
Net proceeds from the sale of real estate								580,000,000
Income tax on gain on sale								$ 484,000,000
New York Segment [Member] | 866 UN Plaza [Member] | Discontinued Operations [Member]
Dispositions [Abstract]
Square Footage Of Real Estate Property | ft²												360
Gross proceeds from the sale of real estate												$ 200,000,000
Net gains on sale of real estate												127,512,000
Net proceeds from the sale of real estate												$ 146,439,000