Related Party Transactions (Details)	1 Months Ended					3 Months Ended	12 Months Ended
	Nov. 30, 2017 USD ($)	Aug. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) unit	Mar. 31, 2015 USD ($)	Jan. 31, 2013 USD ($)	Mar. 31, 2017 USD ($) unit	Dec. 31, 2017 USD ($) investment unit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) investment	Dec. 31, 2010 USD ($)	Nov. 30, 2016 USD ($)
Related Party Transaction [Line Items]
Management and other fees from affiliates							$ 12,600,000	$ 12,400,000	$ 15,600,000
Revenues from development and redevelopment fees							3,000,000	4,200,000	6,700,000
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)							100,926,000	40,970,000
Co-investments							$ 1,155,984,000	1,161,275,000
Number of units acquired | unit							1,328
Encumbrance							$ 2,008,417,000
Payments to acquire equity method investments							293,363,000	183,989,000	127,879,000
Redeemed equity method investment							$ 5,000,000
Number of equity method investments scheduled to mature | investment							2
Gain on disposal of equity method investment							$ 44,837,000	13,046,000	0
Related party transaction, amount of transaction										$ 4,000,000
Limited partnership interest in partnership investments										3.00%
Membership Interest In Sage At Cupertino [Member]
Related Party Transaction [Line Items]
Co-investments			$ 15,300,000			$ 15,300,000
Number of units acquired | unit			230			230
Equity method investment, ownership percentage			40.50%
Contract price			$ 90,000,000
Encumbrance			$ 52,000,000			$ 52,000,000
Notes And Other Receivables From Affiliates [Member]
Related Party Transaction [Line Items]
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)							5,100,000	4,700,000
Related party note receivable, secured, bearing interest at 9.5%, due October 2019 [Member]
Related Party Transaction [Line Items]
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)							6,656,000	6,593,000
Secured Note Receivable, 3.5% Interest Rate, Due December 16, 2017 [Member]
Related Party Transaction [Line Items]
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)							29,500,000	0
Marcus and Millichamp Company TMMC Affiliate [Member]
Related Party Transaction [Line Items]
Preferred equity interest investment in a related party entity					$ 8,600,000
Gain on disposal of equity method investment				$ 500,000
Marcus and Millichamp Company TMMC Affiliate [Member] | Related party note receivable, secured, bearing interest at 9.5%, due October 2019 [Member]
Related Party Transaction [Line Items]
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)											$ 6,600,000
Marcus and Millichamp Company TMMC Affiliate [Member] | Disposition Of Multi-family Properties [Member]
Related Party Transaction [Line Items]
Payments for brokerage fees							0	1,100,000	0
Limited Liability Company [Member]
Related Party Transaction [Line Items]
Payments to acquire equity method investments									$ 20,000,000
Number of equity method investments acquired during the period | investment									3
Cumulative dividend rate (in hundredths)									9.50%
Limited Liability Company [Member] | Related party note receivable, secured, bearing interest at 9.5%, due October 2019 [Member] | Marcus and Millichamp Company TMMC Affiliate [Member]
Related Party Transaction [Line Items]
Notes receivable, related parties							6,700,000	$ 6,600,000
Affiliated Entity [Member] | Secured Note Receivable, 3.5% Interest Rate, Due December 16, 2017 [Member] | Wesco V Property [Member]
Related Party Transaction [Line Items]
Notes receivable, related parties		$ 55,000,000
Notes receivable, related party, interest rate		3.50%
Affiliated Entity [Member] | Secured Note Receivable, 3.5% Interest Rate, Due March 9, 2018 [Member] | BEX III [Member]
Related Party Transaction [Line Items]
Notes and other receivables (includes related party receivables of $41.2 million and $11.3 million as of December 31, 2017 and December 31, 2016, respectively)							$ 29,500,000
Notes receivable, related parties	$ 29,500,000
Notes receivable, related party, interest rate	3.50%