Real Estate Investments - Held for Sale (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) a parcel	Dec. 31, 2019 USD ($) parcel
ASSETS
Operating properties	$ 327,286	$ 0
Accumulated depreciation	(957,189)	(973,318)
Operating real estate investments, net	2,714,647	3,054,797
Construction-in-progress	235,475	180,718
Land held for development	112,420	96,124
Total real estate investments, net	3,101,931	3,371,231
Deferred costs, net	87,811	95,560
Intangible assets, net	55,262	84,851
Other assets	117,959	115,678
Total assets held for sale, net	4,022,770	4,075,969
LIABILITIES
Mortgage notes payable, net	89,435	313,812
Deferred income, gains and rent	24,984	35,284
Intangible liabilities, net	18,577	22,263
Other liabilities	32,104	15,985
Total liabilities held for sale, net	2,451,921	$ 2,387,666
Held-for-sale
ASSETS
Operating properties	327,286
Accumulated depreciation	(59,519)
Operating real estate investments, net	267,767
Construction-in-progress	1,881
Land held for development	7,349
Total real estate investments, net	276,997
Deferred costs, net	10,888
Intangible assets, net	12,497
Other assets	101
Total assets held for sale, net	300,483
LIABILITIES
Mortgage notes payable, net	220,720
Deferred income, gains and rent	4,119
Intangible liabilities, net	613
Other liabilities	541
Total liabilities held for sale, net	225,993
Held-for-sale | Other
LIABILITIES
Number of parcels of land | parcel		2
Held-for-sale | Philadelphia - Office
ASSETS
Operating properties	327,286
Accumulated depreciation	(59,519)
Operating real estate investments, net	267,767
Construction-in-progress	1,881
Land held for development	0
Total real estate investments, net	269,648
Deferred costs, net	10,888
Intangible assets, net	12,497
Other assets	101
Total assets held for sale, net	293,134
LIABILITIES
Mortgage notes payable, net	220,720
Deferred income, gains and rent	4,119
Intangible liabilities, net	613
Other liabilities	541
Total liabilities held for sale, net	225,993
Held-for-sale | Other - Land
ASSETS
Operating properties	0
Accumulated depreciation	0
Operating real estate investments, net	0
Construction-in-progress	0
Land held for development	7,349
Total real estate investments, net	7,349
Deferred costs, net	0
Intangible assets, net	0
Other assets	0
Total assets held for sale, net	7,349
LIABILITIES
Mortgage notes payable, net	0
Deferred income, gains and rent	0
Intangible liabilities, net	0
Other liabilities	0
Total liabilities held for sale, net	$ 0
Held-for-sale | Other - Land | Other
LIABILITIES
Number of parcels of land | parcel	2
Area of land (in acres) | a	35.2