Schedule - Summary of Real Estate and Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate and Accumulated Depreciation [Line Items]
Aggregate cost basis for federal income tax purposes	$ 3,174,700	$ 3,303,200
Initial Cost of Land	463,819
Initial Cost of Building & Improvements	1,722,837
Cost of Improvements Net of Retirements and Impairment Charges	1,113,046
Balance of Land and Land Held for Develop- ment	491,080
Balance of Building & Improvements and Construction in Progress	2,808,622
Accumulated Depreciation Balance	1,111,007	1,060,845	$ 1,015,647	$ 1,061,051
Current Encumbrance(1)	1,059,439
Real estate, federal income tax depreciated basis	2,284,000	2,380,800
Capital City Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	11,642
Initial Cost of Building & Improvements	65,575
Cost of Improvements Net of Retirements and Impairment Charges	49,835
Balance of Land and Land Held for Develop- ment	11,684
Balance of Building & Improvements and Construction in Progress	115,368
Accumulated Depreciation Balance	43,437
Current Encumbrance(1)	$ 59,980
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Cherry Hill Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 29,938
Initial Cost of Building & Improvements	185,611
Cost of Improvements Net of Retirements and Impairment Charges	264,074
Balance of Land and Land Held for Develop- ment	48,608
Balance of Building & Improvements and Construction in Progress	431,015
Accumulated Depreciation Balance	229,299
Current Encumbrance(1)	$ 281,237
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Cumberland Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 8,711
Initial Cost of Building & Improvements	43,889
Cost of Improvements Net of Retirements and Impairment Charges	29,885
Balance of Land and Land Held for Develop- ment	9,842
Balance of Building & Improvements and Construction in Progress	72,643
Accumulated Depreciation Balance	25,478
Current Encumbrance(1)	$ 45,205
Date of Acquisition/ Construction	2005
Life of Depre- ciation	40 years
Dartmouth Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 7,015
Initial Cost of Building & Improvements	28,328
Cost of Improvements Net of Retirements and Impairment Charges	40,919
Balance of Land and Land Held for Develop- ment	7,004
Balance of Building & Improvements and Construction in Progress	69,258
Accumulated Depreciation Balance	37,209
Current Encumbrance(1)	$ 61,125
Date of Acquisition/ Construction	1998
Life of Depre- ciation	40 years
Exton Square Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 21,460
Initial Cost of Building & Improvements	121,326
Cost of Improvements Net of Retirements and Impairment Charges	45,127
Balance of Land and Land Held for Develop- ment	26,299
Balance of Building & Improvements and Construction in Progress	161,614
Accumulated Depreciation Balance	53,683
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Francis Scott Key Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 9,786
Initial Cost of Building & Improvements	47,526
Cost of Improvements Net of Retirements and Impairment Charges	42,545
Balance of Land and Land Held for Develop- ment	9,440
Balance of Building & Improvements and Construction in Progress	90,417
Accumulated Depreciation Balance	37,827
Current Encumbrance(1)	$ 68,469
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Jacksonville Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 9,974
Initial Cost of Building & Improvements	47,802
Cost of Improvements Net of Retirements and Impairment Charges	29,759
Balance of Land and Land Held for Develop- ment	9,974
Balance of Building & Improvements and Construction in Progress	77,561
Accumulated Depreciation Balance	35,988
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Magnolia Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 9,279
Initial Cost of Building & Improvements	44,165
Cost of Improvements Net of Retirements and Impairment Charges	44,663
Balance of Land and Land Held for Develop- ment	15,933
Balance of Building & Improvements and Construction in Progress	82,174
Accumulated Depreciation Balance	44,339
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	1998
Life of Depre- ciation	40 years
Monroe Marketplace [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 1,177
Initial Cost of Building & Improvements	0
Cost of Improvements Net of Retirements and Impairment Charges	0
Balance of Land and Land Held for Develop- ment	1,177
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2006
Monroe Marketplace [Member]
Real Estate and Accumulated Depreciation [Line Items]
Balance of Building & Improvements and Construction in Progress	$ 0
Accumulated Depreciation Balance	$ 0
Life of Depre- ciation	10 years
Moorestown Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 11,368
Initial Cost of Building & Improvements	62,995
Cost of Improvements Net of Retirements and Impairment Charges	87,548
Balance of Land and Land Held for Develop- ment	11,368
Balance of Building & Improvements and Construction in Progress	150,543
Accumulated Depreciation Balance	58,869
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Patrick Henry Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 16,075
Initial Cost of Building & Improvements	86,643
Cost of Improvements Net of Retirements and Impairment Charges	51,380
Balance of Land and Land Held for Develop- ment	16,397
Balance of Building & Improvements and Construction in Progress	137,701
Accumulated Depreciation Balance	65,052
Current Encumbrance(1)	$ 92,398
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Plymouth Meeting Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 29,265
Initial Cost of Building & Improvements	58,388
Cost of Improvements Net of Retirements and Impairment Charges	109,234
Balance of Land and Land Held for Develop- ment	29,958
Balance of Building & Improvements and Construction in Progress	166,929
Accumulated Depreciation Balance	81,344
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Mall At Prince Georges [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 13,065
Initial Cost of Building & Improvements	57,686
Cost of Improvements Net of Retirements and Impairment Charges	56,723
Balance of Land and Land Held for Develop- ment	13,066
Balance of Building & Improvements and Construction in Progress	114,408
Accumulated Depreciation Balance	54,591
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	1998
Life of Depre- ciation	40 years
Springfield Town Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 119,912
Initial Cost of Building & Improvements	353,551
Cost of Improvements Net of Retirements and Impairment Charges	17,547
Balance of Land and Land Held for Develop- ment	119,912
Balance of Building & Improvements and Construction in Progress	371,098
Accumulated Depreciation Balance	36,204
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2015
Life of Depre- ciation	40 years
Sunrise Plaza Land [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 395
Initial Cost of Building & Improvements	0
Cost of Improvements Net of Retirements and Impairment Charges	(29)
Balance of Land and Land Held for Develop- ment	366
Balance of Building & Improvements and Construction in Progress	0
Accumulated Depreciation Balance	0
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2005
Life of Depre- ciation	0 years
Swedes Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 189
Initial Cost of Building & Improvements	0
Cost of Improvements Net of Retirements and Impairment Charges	36
Balance of Land and Land Held for Develop- ment	225
Balance of Building & Improvements and Construction in Progress	0
Accumulated Depreciation Balance	0
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2004
Life of Depre- ciation	0 years
Valley Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 13,187
Initial Cost of Building & Improvements	60,658
Cost of Improvements Net of Retirements and Impairment Charges	50,800
Balance of Land and Land Held for Develop- ment	15,591
Balance of Building & Improvements and Construction in Progress	109,054
Accumulated Depreciation Balance	43,131
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Valley View Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 9,880
Initial Cost of Building & Improvements	46,817
Cost of Improvements Net of Retirements and Impairment Charges	(2,144)
Balance of Land and Land Held for Develop- ment	9,077
Balance of Building & Improvements and Construction in Progress	45,476
Accumulated Depreciation Balance	10,108
Current Encumbrance(1)	$ 28,623
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Viewmont Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 12,505
Initial Cost of Building & Improvements	61,519
Cost of Improvements Net of Retirements and Impairment Charges	45,676
Balance of Land and Land Held for Develop- ment	12,725
Balance of Building & Improvements and Construction in Progress	106,975
Accumulated Depreciation Balance	40,593
Current Encumbrance(1)	$ 57,000
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Willow Grove Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 26,748
Initial Cost of Building & Improvements	131,189
Cost of Improvements Net of Retirements and Impairment Charges	86,228
Balance of Land and Land Held for Develop- ment	36,188
Balance of Building & Improvements and Construction in Progress	207,977
Accumulated Depreciation Balance	96,756
Current Encumbrance(1)	$ 163,224
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
Woodland Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 35,540
Initial Cost of Building & Improvements	124,504
Cost of Improvements Net of Retirements and Impairment Charges	73,883
Balance of Land and Land Held for Develop- ment	44,196
Balance of Building & Improvements and Construction in Progress	189,731
Accumulated Depreciation Balance	69,121
Current Encumbrance(1)	$ 127,200
Date of Acquisition/ Construction	2005
Life of Depre- ciation	40 years
Wyoming Valley Mall [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 14,153
Initial Cost of Building & Improvements	73,035
Cost of Improvements Net of Retirements and Impairment Charges	31,588
Balance of Land and Land Held for Develop- ment	13,302
Balance of Building & Improvements and Construction in Progress	105,474
Accumulated Depreciation Balance	47,978
Current Encumbrance(1)	$ 74,978
Date of Acquisition/ Construction	2003
Life of Depre- ciation	40 years
White Clay Point [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 31,000
Initial Cost of Building & Improvements	11,803
Cost of Improvements Net of Retirements and Impairment Charges	(28,803)
Balance of Land and Land Held for Develop- ment	10,914
Balance of Building & Improvements and Construction in Progress	3,086
Accumulated Depreciation Balance	0
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2005
Life of Depre- ciation	0 years
Springhills [Member]
Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Land	$ 21,555
Initial Cost of Building & Improvements	9,827
Cost of Improvements Net of Retirements and Impairment Charges	(13,428)
Balance of Land and Land Held for Develop- ment	17,834
Balance of Building & Improvements and Construction in Progress	120
Accumulated Depreciation Balance	0
Current Encumbrance(1)	$ 0
Date of Acquisition/ Construction	2006
Life of Depre- ciation	0 years
Mortgage Loan [Member]
Real Estate and Accumulated Depreciation [Line Items]
Debt Issuance Costs, Net	$ 3,400	$ 4,500