Provision for Impairment of Investment Properties - Impairment Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) ft²
Impaired Long-Lived Assets Held and Used [Line Items]
Provision for impairment of investment properties	$ 12,298	$ 2,079	$ 67,003
Estimated fair value of impaired properties as of impairment date	$ 16,944	$ 85,321	$ 107,400
Streets of Yorktown
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²	85,200
Provision for impairment of investment properties	$ 11,177
King Philip's Crossing
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²	105,900
Provision for impairment of investment properties	$ 1,121
Schaumburg Towers
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²		895,400	895,400
Provision for impairment of investment properties		$ 1,116	$ 45,638
CVS Pharmacy - Lawton, OK
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²		10,900
Provision for impairment of investment properties		$ 200
Orange Plaza (Golfland Plaza)
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²		58,200
Provision for impairment of investment properties		$ 763
Century III Plaza, excluding the Home Depot parcel
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²			152,200
Provision for impairment of investment properties			$ 3,304
Lakepointe Towne Center
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²			196,600
Provision for impairment of investment properties			$ 9,958
Saucon Valley Square
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²			80,700
Provision for impairment of investment properties			$ 184
High Ridge Crossing
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²			76,900
Provision for impairment of investment properties			$ 3,480
Home Depot Plaza
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²			135,600
Provision for impairment of investment properties			$ 4,439