Other Significant Transactions (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Other Significant Transactions [Line Items]
Proceeds from issuance of Common Stock	$ 366,585,000	$ 0
Common Class A [Member]
Other Significant Transactions [Line Items]
Shares of Common Stock Issued (in shares)	9,430,000
Gross Offering Price Per Share Of Common Stock Issued (in dollars per share)	$ 38.90
Class A Cumulative Preferred Stock [Member]
Other Significant Transactions [Line Items]
Par Value Per Share of Common Stock Issued (in dollars per share)	$ 0.01
Series A Community Reinvestment Act Preferred Stock [Member]
Other Significant Transactions [Line Items]
Preferred Stock, Redemption Amount	27,000,000
Napico Portfolio [Member]
Other Significant Transactions [Line Items]
Number of partnerships in legally sold portfolio	17
Napico Portfolio [Member] | Wholly And Partially Owned Consolidated Properties [Member]
Other Significant Transactions [Line Items]
Number of apartment communities	14
Napico Portfolio [Member] | Unconsolidated Properties [Member]
Other Significant Transactions [Line Items]
Number of apartment communities	53
Acquired Apartment Community [Member]
Other Significant Transactions [Line Items]
Apartment Community Acquisition Price	$ 38,300,000
Number of apartment homes in apartment communities	94