Related Party Transactions (Details) $ in Thousands	1 Months Ended							3 Months Ended		9 Months Ended		12 Months Ended			17 Months Ended
	Aug. 31, 2019 USD ($)	Jun. 30, 2019 USD ($) apartment	Feb. 28, 2019 USD ($)	Oct. 31, 2018 USD ($) apartment	May 31, 2018 USD ($) apartment	Nov. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) unit	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($) investment	Sep. 30, 2019 USD ($)	Nov. 30, 2016 USD ($)
Related Party Transaction [Line Items]
Management and other fees from affiliates								$ 3,700	$ 3,600	$ 10,600	$ 10,000
Development and redevelopment fees								1,200	$ 1,300	3,600	3,200
Debt assumed in connection with acquisition										143,006	0
Payments to acquire preferred equity investments										306,233	$ 100,202
Co-investments								1,413,861		1,413,861		$ 1,300,140			$ 1,413,861
Short-term loans outstanding and due from various joint ventures								4,300		$ 4,300		4,500			4,300
Brio, Walnut Creek, California [Member]
Related Party Transaction [Line Items]
Number of units acquired | apartment		300
Payments to acquire real estate		$ 164,900
Debt assumed in connection with acquisition		$ 98,700
Multifamily Development in Mountain View, California [Member]
Related Party Transaction [Line Items]
Preferred returns rate			11.00%
Commitment to fund preferred equity investment			$ 24,500
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
Commitment funded amount															21,900
Membership Interest In Sage At Cupertino [Member]
Related Party Transaction [Line Items]
Number of units acquired | unit							230
Co-investments							$ 15,300
Equity method investment, ownership percentage acquired							40.50%
Contract price							$ 90,000
Mortgage encumbrance							$ 52,000
Related Party Bridge Loan on Property Acquired By BEX IV [Member]
Related Party Transaction [Line Items]
Interest rate										3.25%
Affiliated Entity [Member] | Multifamily Development in Mountain View, California [Member]
Related Party Transaction [Line Items]
Payments to acquire preferred equity investments			$ 24,500
Preferred returns rate			11.00%
Affiliated Entity [Member] | Home Community Development in Burlingame, California [Member]
Related Party Transaction [Line Items]
Number of units acquired | apartment				268
Payments to acquire preferred equity investments				$ 18,600
Preferred returns rate				12.00%
Affiliated Entity [Member] | Related Party Bridge Loans on Property Acquired by Bex III [Member]
Related Party Transaction [Line Items]
Notes receivable						$ 29,500
Interest rate						3.50%
Affiliated Entity [Member] | Related Party Bridge Loans on Property Acquired By Wesco V [Member]
Related Party Transaction [Line Items]
Notes receivable	$ 89,000							89,200		$ 89,200					89,200
Interest rate	1.30%
Affiliated Entity [Member] | Related Party Bridge Loan on Property Acquired By BEX IV [Member]
Related Party Transaction [Line Items]
Notes receivable	$ 44,400							44,500		44,500					44,500
Interest rate	3.25%
Limited Liability Company [Member]
Related Party Transaction [Line Items]
Payments to acquire preferred equity investments														$ 20,000
Number of preferred equity method investments acquired during period (in investments) | investment														3
Preferred stock, stated interest percentage														9.50%
Redeemed amount												5,000	$ 5,000
Limited Liability Company [Member] | Secured Note Receivable, 9.90% Interest Rate, Due October 2019 [Member] | Marcus and Millichamp Company TMMC Affiliate [Member]
Related Party Transaction [Line Items]
Notes receivable								$ 6,600		$ 6,600		$ 6,600			$ 6,600	$ 6,600