Significant Transactions During the Second Quarter of 2015 and Subsequent Events (Details) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended	6 Months Ended
	Mar. 31, 2015 USD ($)	Jun. 30, 2015 USD ($) apartment	Apr. 30, 2015 USD ($) apartment	Jun. 30, 2015 USD ($) apartment $ / shares shares	Jun. 30, 2015 USD ($) apartment	Jun. 30, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments		$ 1,300			$ 97,512	$ 58,029
Apartment units owned | apartment		58,768		58,768	58,768
Net proceeds from issuance of common stock					$ 272,664	$ 278,334
Common Stock [Member]
Schedule of Equity Method Investments [Line Items]
Sale of common stock (in shares) | shares				396,840
Average share price (in dollars per share) | $ / shares				$ 223.59
Net proceeds from issuance of common stock				$ 88,500
The Reveal [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage increase during period			49.50%
Number of units acquired | apartment			438
Contract price			$ 73,000
Membership interest in Wesco I [Member]
Schedule of Equity Method Investments [Line Items]
Preferred equity investments	50.00%
Gain on remeasurement of investment	$ 12,700				$ 12,700
Avant I [Member]
Schedule of Equity Method Investments [Line Items]
Number of units acquired | apartment		247
Contract price		$ 99,000
Greentree Apartments [Member] | Limited Liability Company [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments		$ 10,000
Apartment units owned | apartment		220		220	220
Preferred stock, stated interest percentage		9.50%
Sterling Cove [Member] | Limited Liability Company [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments		$ 5,000
Apartment units owned | apartment		218		218	218
Preferred stock, stated interest percentage		9.50%