Significant Transactions During the Nine Months Ended September 30, 2018 and Subsequent Events (Details)	1 Months Ended					5 Months Ended	9 Months Ended
	Oct. 31, 2018 USD ($) unit apartment	Jun. 30, 2018 USD ($) unit	May 31, 2018 USD ($) apartment	Mar. 31, 2018 USD ($)	Jan. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Other Commitments [Line Items]
Promote income				$ 20,500,000
Proceeds from full redemption of a preferred equity investment in a joint venture		$ 26,500,000			$ 2,400,000
Gain on early redemption of preferred equity investment		$ 1,600,000
Stock repurchased and retired during the period (in shares) | shares					16,834
Stock repurchased and retired during the period					$ 3,800,000
Purchase authority remaining under stock repurchase plan						$ 245,200,000	$ 245,200,000
Acquisitions of real estate							7,807,000	$ 200,028,000
Preferred equity investments							$ 100,202,000	$ 231,552,000
Senior Notes [Member] | Senior Unsecured Notes At 4.500% [Member]
Other Commitments [Line Items]
Debt, face amount				$ 300,000,000
Debt, term (in years)				30 years
Debt, stated interest rate				4.50%
Domain [Member]
Other Commitments [Line Items]
Number of units sold | unit		379
Proceeds from sale of real estate		$ 132,000,000
Gain (loss) on sale of properties		$ 22,200,000
Apartment Home Community In Ventura, California [Member]
Other Commitments [Line Items]
Commitment to fund preferred equity investment			$ 26,500,000
Number of units acquired | apartment			400
Preferred returns rate			10.25%
Commitment funded amount						$ 20,600,000
BEX III [Member]
Other Commitments [Line Items]
Repayments of related party debt					$ 29,500,000
Meridian at Midtown [Member] | Wesco V [Member] | Subsequent Event [Member]
Other Commitments [Line Items]
Number of units acquired | unit	218
Acquisitions of real estate	$ 104,000,000
Mortgage debt assumed in connection with the acquisition	$ 69,900,000
Effective interest rate of mortgage debt incurred as consideration	4.50%
Home Community Development in Burlingame, California [Member] | Subsequent Event [Member] | Affiliated Entity [Member]
Other Commitments [Line Items]
Number of units acquired | apartment	268
Preferred returns rate	12.00%
Preferred equity investments	$ 18,600,000