Investments in Partially Owned Entities - Consolidated Variable Interest Entities ("VIEs") - Additional Information (Details) $ in Thousands	Sep. 30, 2018 USD ($) Property ApartmentUnit	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Properties | Property	306
Apartment units | ApartmentUnit	79,260
Total assets	$ 20,542,150	$ 20,570,599
Total liabilities	$ 9,693,820	9,729,781
Consolidated VIE [Member]
Variable Interest Entity [Line Items]
Properties | Property	17
Apartment units | ApartmentUnit	3,535
Total assets	$ 714,400	518,900
Total liabilities	$ 315,600	$ 307,000