Significant Transactions During the Third Quarter of 2015 and Subsequent Events (Details) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended	9 Months Ended
	Oct. 30, 2015 USD ($) $ / shares shares	Aug. 31, 2015 USD ($) apartment	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) apartment $ / shares shares	Sep. 30, 2015 USD ($) apartment shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments			$ 1,300		$ 119,120	$ 128,268
Apartment units owned | apartment				58,768	58,768
Co-investments		$ 20,400		$ 1,036,043	$ 1,036,043		$ 1,042,423
Proceeds from redemption of preferred equity interest		1,500
Net proceeds from issuance of common stock					307,835	$ 449,464
Sale of common stock					$ 307,835
Common Stock [Member]
Schedule of Equity Method Investments [Line Items]
Sale of common stock (in shares) | shares				155,728	1,382,000
Average share price (in dollars per share) | $ / shares				$ 228.61
Net proceeds from issuance of common stock				$ 35,400
Common Stock [Member] | Subsequent Event [Member]
Schedule of Equity Method Investments [Line Items]
Sale of common stock (in shares) | shares	107,098
Average share price (in dollars per share) | $ / shares	$ 228.61
Sale of common stock	$ 24,300
Alta Vista Apartments [Member] | Limited Liability Company [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments		$ 5,000
Apartment units owned | apartment		92
Preferred stock, stated interest percentage		9.50%