Dispositions (Narrative) (Details) $ / shares in Units, ft² in Thousands										1 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Jul. 17, 2017 USD ($) ft² development_asset myAptUnit myProperty	Dec. 21, 2016 USD ($)	Dec. 22, 2015 USD ($) ft²	Sep. 09, 2015 USD ($) ft²	Aug. 06, 2015 USD ($)	Mar. 31, 2015 USD ($) ft² shares	Mar. 13, 2015 USD ($)	Jan. 15, 2015 USD ($) property	Jul. 17, 2017 USD ($) ft² development_asset myAptUnit myProperty	Jun. 20, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($) ft²	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Jun. 30, 2016	Mar. 15, 2016 USD ($)
Dispositions
(Loss) income from discontinued operations														$ (13,228,000)	$ 404,912,000	$ 223,511,000
Net gains on sale of real estate and a lease position														6,605,000	5,074,000	167,801,000
Cash, cash equivalents, restricted cash and restricted cash equivalents, included in spin-off														416,237,000	0	0
Real estate impairment losses														(501,000)	(175,735,000)	(251,821,000)
Assets related to discontinued operations														1,357,000	3,568,613,000
Liabilities related to discontinued operations														3,620,000	1,259,443,000
Cash considerations transferred during spin off (in usd)														0	0	234,967,000
Operating partnership units, value														$ 0	0	$ 80,000,000
PREIT
Dispositions
Operating partnership units received from buyer | shares														6,250,000
Washington DC
Dispositions
Pro rata basis of distribution														.5
Washington DC | Skyline Properties
Dispositions
Mortgage loan outstanding																		$ 678,000,000
Real estate impairment losses												$ 160,700,000
Capitalization rate												8.00%
Discount rates												8.20%
Interest expense															7,823,000
Assets related to discontinued operations			$ 236,535,000
Liabilities related to discontinued operations			724,412,000
Gain on disposition of assets			$ 487,877,000
Discontinued Operations | Urban Edge Properties
Dispositions
Cash considerations transferred during spin off (in usd)									$ 225,000,000
Discontinued Operations | Urban Edge Properties | Strip Shopping Centers
Dispositions
Number of real estate properties | property									79
Discontinued Operations | Urban Edge Properties | Regional Malls
Dispositions
Number of real estate properties | property									3
Discontinued Operations | Geary Street, CA
Dispositions
Proceeds from sale of interest in leases								$ 34,189,000
Gain on sale of interest in leases								$ 21,376,000
Discontinued Operations | Springfield Mall | PREIT
Dispositions
Square footage of real estate property | ft²							1,350						1,350
Gross proceeds from the sale of real estate							$ 485,313,000
Net gains on sale of real estate and a lease position													$ 7,192,000		$ 7,823,000
Cash Proceeds from sale of real estate							$ 340,000,000
Operating partnership units received from buyer | shares							6,250,000
Operating partnership units, value							$ 145,313,000
Operating partnership units value per unit | $ / shares															$ 23.25
Deferred gain on sale of property															$ 631,000
Discontinued Operations | Monmouth Mall | Regional Malls
Dispositions
Gross proceeds from the sale of real estate						$ 38,000,000
Net gains on sale of real estate and a lease position						$ 33,153,000
Sale of ownership interest percent						50.00%
Market value of the property sold						$ 229,000,000
Discontinued Operations | New York | 20 Broad Street | Office
Dispositions
Square footage of real estate property | ft²				473
Gross proceeds from the sale of real estate				$ 200,000,000
(Loss) income from discontinued operations				157,000,000
Net gains on sale of real estate and a lease position				142,000,000
Lease termination income				$ 15,000,000
Discontinued Operations | Washington DC | 1750 Pennsylvania Avenue | Office
Dispositions
Square footage of real estate property | ft²					278
Gross proceeds from the sale of real estate					$ 182,000,000
Gain (loss) on disposition of intangible assets					102,000,000
Deferred tax gain on sale of real estate					$ 137,000,000
Discontinued Operations | Estimate | Springfield Mall | PREIT
Dispositions
Capitalization rate	5.50%
Additional consideration receivable, percent	50.00%
Additional consideration threshold, value	$ 465,000,000
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC
Dispositions
Disposal group, including discontinued operation, cash and proceeds from refinancing		$ 275,000,000								$ 275,000,000
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC | Office
Dispositions
Number of real estate properties | myProperty		37								37
Net rentable area | ft²		11,100								11,100
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC | Multifamily
Dispositions
Number of real estate properties | myProperty		5								5
Number of units in real estate property (in number of residential units) | myAptUnit		3,133								3,133
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC | Other Property
Dispositions
Number of real estate properties | myProperty		5								5
Net rentable area | ft²		406								406
Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC | Future development assets
Dispositions
Square footage of real estate property | ft²		10,400								10,400
Number of development assets | development_asset		18								18
The Bartlett
Dispositions
Loans payable											$ 220,000,000
Debt term											5 years
The Bartlett | Washington DC
Dispositions
Cash, cash equivalents, restricted cash and restricted cash equivalents, included in spin-off		$ 412,500,000
LIBOR | The Bartlett
Dispositions
Spread Over LIBOR (in percentage)											1.70%
JBGS | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC
Dispositions
Repayments of debt		$ 43,581,000
JBGS | The Bartlett | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC
Dispositions
Net proceeds from debt										$ 217,000,000
JBGS | LIBOR | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | Washington DC
Dispositions
Spread Over LIBOR (in percentage)		1.25%
Minimum
Dispositions
Weighted average interest rate																	3.00%
Maximum
Dispositions
Weighted average interest rate																	4.50%