Investments in Partially Owned Entities - Summary of Companys Investment in Unconsolidated Entities (Parenthetical) (Details)	12 Months Ended
	Dec. 31, 2018 ApartmentUnit JointVenture	Sep. 30, 2019 ApartmentUnit
Variable Interest Entity [Line Items]
Apartment units		80,299
Operating Property [Member] | Unconsolidated Non-VIE [Member]
Variable Interest Entity [Line Items]
Apartment units	945
Number of joint ventures under agreements | JointVenture	2