Provision for Impairment of Investment Properties - Impairment Charges (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($) ft²	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($) ft²
Impaired Long-Lived Assets Held and Used [Line Items]
Provision for impairment of investment properties	$ 724	$ 13,034	$ 1,316	$ 13,034
Estimated fair value of impaired properties as of impairment date			$ 76,871	$ 22,500
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
CVS Pharmacy - Lawton, OK
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²	10,900		10,900
Provision for impairment of investment properties			$ 200
Century III Plaza, excluding the Home Depot parcel
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²		152,200		152,200
Provision for impairment of investment properties				$ 3,076
Lakepointe Towne Center
Impaired Long-Lived Assets Held and Used [Line Items]
Square footage | ft²		196,600		196,600
Provision for impairment of investment properties				$ 9,958