Real Estate Disposition Activities (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Jun. 30, 2014 community	Dec. 31, 2014 home community	Dec. 31, 2013 home	Dec. 31, 2012 home
Summary of income from discontinued operations
Number of communities sold	8	4
Apartment homes		1,337	3,299	1,578
Sale of Communities Gross Sales Price		$ 296,200
Gain (loss) on sale of real estate assets		255,300	278,471	146,591
Number of apartment communities sold that were owned through a TRS		1
Income tax expense		9,368	0	0
Repayments of secured mortgages		16,341	0	0
Gross sales price		304,250	932,800	280,550
Net proceeds		281,125	919,442	274,018
Number of communities held for sale		1
Avalon Valley
Summary of income from discontinued operations
Apartment homes		268
Gross sales price		53,325
Net proceeds		52,147
Oakwood Philadelphia
Summary of income from discontinued operations
Apartment homes		80
Repayments of secured mortgages		16,341
Gross sales price		28,875
Net proceeds		10,932
Avalon Danvers
Summary of income from discontinued operations
Apartment homes		433
Gross sales price		108,500
Net proceeds		107,231
Archstone Memorial Heights
Summary of income from discontinued operations
Apartment homes		556
Gross sales price		105,500
Net proceeds		103,182
Avalon Hingham MA and Brooklyn NY
Summary of income from discontinued operations
Net proceeds			1,313
Huntington Station Land
Summary of income from discontinued operations
Gain (loss) on sale of real estate assets		490
Gross sales price		8,050
Net proceeds		7,633
Mortgage notes payable
Summary of income from discontinued operations
Repayments of secured mortgages	5,914
Mortgage notes payable | Notes payable maturing in 2015
Summary of income from discontinued operations
Repayments of secured mortgages	10,427
Wholly Owned Properties
Summary of income from discontinued operations
Gain (loss) on sale of real estate assets		$ 106,138