Significant Transactions During the Second Quarter of 2014 and Subsequent Events (Details) (USD $)	0 Months Ended	1 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended	1 Months Ended
	Nov. 05, 2014	Oct. 31, 2014	Sep. 30, 2014	Sep. 30, 2013	Nov. 04, 2014	Sep. 30, 2014 Common Stock [Member]	Sep. 30, 2014 Common Stock [Member]	Jul. 31, 2014 Paragon Apartments [Member] Apartment	Aug. 31, 2014 Apex [Member] Unit	Aug. 31, 2014 Ellington [Member] Unit
Acquisitions [Abstract]
Cost of acquired entity		$ 180,000,000	$ 555,826,000	$ 0				$ 111,000,000	$ 150,000,000	$ 58,800,000
Fixed rate of interest (in hundredths)		5.50%
Term of loan		Feb. 28, 2017
Interest Expense Attributable to the Entity		110,000,000
Number of units								301	366	220
Ownership percentage (in hundredths)		50.00%	50.00%		28.20%
Income from Promotion	5,500,000
Redemption Penalties		5,300,000
Proceed from sale of assets	23,500,000	101,000,000
Common Stock [Abstract]
Shares issued during the period (in shares)						801,909	2,527,000
Average share price (in dollars per share)						$ 190.06	$ 177.83
Proceed from stock issuance, net of commissions			$ 450,812,000	$ 122,905,000		$ 151,400,000	$ 450,800,000