Related Party Transactions (Details)	1 Months Ended				3 Months Ended		5 Months Ended	9 Months Ended		12 Months Ended
	Oct. 31, 2018 USD ($) apartment	May 31, 2018 USD ($) apartment	Nov. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) unit	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) investment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($) investment	Nov. 30, 2016 USD ($)
Related Party Transaction [Line Items]
Management and other fees from affiliates					$ 3,600,000	$ 3,100,000		$ 10,000,000	$ 9,000,000
Development and redevelopment fees					1,300,000	$ 700,000		3,200,000	2,100,000
Payments to acquire preferred equity investments								100,202,000	$ 231,552,000
Co-investments					1,267,593,000		$ 1,267,593,000	$ 1,267,593,000		$ 1,155,984,000
Equity investment, redeemed amount										5,000,000
Number of equity method investments scheduled to mature | investment								2
Short-term loans outstanding and due from various joint ventures					3,600,000		3,600,000	$ 3,600,000		5,100,000
Apartment Home Community In Ventura, California [Member]
Related Party Transaction [Line Items]
Number of units acquired | apartment		400
Preferred returns rate		10.25%
Commitment to fund preferred equity investment		$ 26,500,000
Commitment funded amount							20,600,000
Membership Interest In Sage At Cupertino [Member]
Related Party Transaction [Line Items]
Number of units acquired | unit				230
Co-investments				$ 15,300,000
Equity method investment, ownership percentage acquired				40.50%
Contract price				$ 90,000,000
Mortgage encumbrance				$ 52,000,000
Affiliated Entity [Member] | Subsequent Event [Member] | Home Community Development in Burlingame, California [Member]
Related Party Transaction [Line Items]
Payments to acquire preferred equity investments	$ 18,600,000
Number of units acquired | apartment	268
Preferred returns rate	12.00%
Affiliated Entity [Member] | BEX III [Member] | Secured Note Receivable, 3.5% Interest Rate, Due March 9, 2018 [Member]
Related Party Transaction [Line Items]
Notes receivable			$ 29,500,000		0		0	0
Note receivable, related party, interest rate			3.50%
Limited Liability Company [Member]
Related Party Transaction [Line Items]
Payments to acquire preferred equity investments											$ 20,000,000
Number of preferred equity method investments acquired during period (in investments) | investment											3
Preferred stock, stated interest percentage											9.50%
Limited Liability Company [Member] | Marcus and Millichamp Company TMMC Affiliate [Member] | Secured Note Receivable, 9.50% Interest Rate, Due October 2019 [Member]
Related Party Transaction [Line Items]
Notes receivable					$ 6,600,000		$ 6,600,000	$ 6,600,000		$ 6,700,000		$ 6,600,000