Schedule III - Real Estate and Accumulated Depreciation (Reconciliation of Real Estate Investments and Accumulated Depreciation) (Parentheticals) (Details)		12 Months Ended
		Dec. 31, 2018 USD ($) Parcel	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the Beginning of the Year		$ 3,832,348,000	$ 3,659,886,000	$ 4,487,588,000
Additions:
Acquisitions		509,654,000	62,586,000	0
Capital expenditures and assets placed into service		129,274,000	356,857,000	213,996,000
Less:
Dispositions/impairments/placed into redevelopment		(469,441,000)	(189,395,000)	(962,676,000)
Retirements		(48,516,000)	(57,586,000)	(79,022,000)
Balance at the end of the year		3,953,319,000	3,832,348,000	3,659,886,000
Assets held for sale	[1]	0	0	(73,591,000)
Operating properties		3,953,319,000	3,832,348,000	3,586,295,000
Tax Basis of Assets, Cost for Income Tax Purposes		3,300,000,000	3,100,000,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the beginning of the year		895,091,000	885,392,000	1,080,616,000
Real Estate Accumulated Depreciation Additions [Abstract]
Depreciation expense		135,473,000	135,822,000	131,859,000
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Dispositions/impairments/placed into redevelopment		(117,589,000)	(74,178,000)	(250,110,000)
Retirements		(47,513,000)	(51,945,000)	(76,973,000)
Balance at the end of the year		865,462,000	895,091,000	885,392,000
Assets held for sale		0	0	(32,916,000)
Real Estate Investment Property, Accumulated Depreciation		$ 865,462,000	895,091,000
Land Parcel One [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Lease agreement term		99 years
Land Parcel Two [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Lease agreement term		99 years
Leashold Interest Land [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Number of Parcels | Parcel		2
Philadelphia CBD [Member] | 618-634 Market Street [Member]
Less:
Balance at the end of the year		$ 21,108,000.00
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Balance at the end of the year		$ 4,601,000
Useful life		5 years
Philadelphia CBD [Member] | 618-634 Market Street [Member] | Parking Structure [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Useful life		5 years
Accumulated Depreciation, including HFS [Member]
Real Estate Accumulated Depreciation Less Dispositions [Abstract]
Real Estate Investment Property, Accumulated Depreciation		$ 865,462,000	$ 895,091,000	$ 852,476,000

[1] As of December 31, 2016, three office properties in the Metropolitan Washington, D.C. segment and two office properties in the Other segment were classified as held for sale in accordance with applicable accounting standards for long-lived assets. See the “2016” section of Note 3, “Real Estate Investments,” for further information.