Other Significant Transactions (Details Textual) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) Units Property $ / shares shares	Sep. 30, 2014 USD ($)
Other Significant Transactions [Line Items]
Proceeds from issuance of Common Stock	$ 366,580	$ 0
Preferred Stock, Redemption Amount	$ 27,000	$ 9,516
Common Class A [Member]
Other Significant Transactions [Line Items]
Shares of Common Stock Issued (in shares) | shares	9,430,000
Gross Offering Price Per Share Of Common Stock Issued (in dollars per share) | $ / shares	$ 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) | $ / shares	$ 0.01
Series A Community Reinvestment Act Preferred Stock [Member]
Other Significant Transactions [Line Items]
Preferred Stock, Redemption Amount	$ 27,000
Napico Portfolio [Member]
Other Significant Transactions [Line Items]
Number of partnerships in legally sold portfolio	14
Napico Portfolio [Member] | Wholly And Partially Owned Consolidated Properties [Member]
Other Significant Transactions [Line Items]
Number of apartment communities | Property	12
Napico Portfolio [Member] | Unconsolidated Properties [Member]
Other Significant Transactions [Line Items]
Number of apartment communities | Property	47
Mezzo - Atlanta GA [Member] [Member]
Other Significant Transactions [Line Items]
Apartment Community Acquisition Price	$ 38,300
Number of apartment homes in apartment communities | Units	94
Axiom Apartment Homes - Cambridge MA [Member]
Other Significant Transactions [Line Items]
Apartment Community Acquisition Price	$ 63,000
Number of apartment homes in apartment communities | Units	115
Percentage of Apartment Homes Occupied	77.00%
270 on Third - Cambridge MA [Member]
Other Significant Transactions [Line Items]
Payments to Acquire Real Estate	$ 27,900
Expected Investment in Development Project	45,000
Development in Process	$ 38,800
Number of apartment homes in apartment communities | Units	91