Investments in Real Estate Entities (Details 2)		3 Months Ended		12 Months Ended	15 Months Ended
		Mar. 31, 2016 USD ($) community home	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) land_parcel	Mar. 31, 2016 USD ($) community	Jan. 31, 2016 USD ($)
Assets:
Real estate, net		$ 1,111,906,000		$ 1,392,833,000	$ 1,111,906,000
Other assets		59,975,000		57,044,000	59,975,000
Total assets		1,171,881,000		1,449,877,000	1,171,881,000
Liabilities and partners' capital:
Mortgage notes payable and credit facility		782,988,000		947,205,000	782,988,000
Other liabilities		21,250,000		20,471,000	21,250,000
Partners' capital		367,643,000		482,201,000	367,643,000
Total liabilities and partners' capital		1,171,881,000		1,449,877,000	1,171,881,000
Combined summary of the operating results of the accounted for using the equity method
Rental and other income		36,955,000	$ 45,255,000
Operating and other expenses		(14,170,000)	(17,337,000)
Gain on sale of communities		103,321,000	32,490,000
Interest expense, net		(20,001,000)	(10,477,000)
Depreciation expense		(9,240,000)	(11,902,000)
Net income		96,865,000	38,029,000
Costs in excess of equity in underlying net assets of the respective investments		39,871,000		$ 40,978,000	$ 39,871,000
Equity in income of unconsolidated real estate entities		27,969,000	34,566,000
Business Combination, Acquisition Related Costs, Abandoned Pursuit Costs		1,846,000	1,187,000
Casualty and Impairment Loss (Gain)		$ (2,202,000)	5,788,000
Number of Land Parcels Impaired | land_parcel				2
Number of Land Parcels Held For Sale | community		3			3
Other than Temporary Impairment Losses, Investments		$ 0	0
Real Estate Investment Property, Net		$ 14,226,433,000		$ 13,834,297,000	$ 14,226,433,000
Number of Communities Acquired | community		2
Avalon Hoboken [Member]
Combined summary of the operating results of the accounted for using the equity method
Number of Apartment Homes, Acquired | home		217
Business Combination, Consideration Transferred		$ 129,700,000
Avalon Potomac Yard [Member]
Combined summary of the operating results of the accounted for using the equity method
Number of Apartment Homes, Acquired | home		323
Business Combination, Consideration Transferred		$ 108,250,000
Funds [Member]
Equity method investment
Gains (Losses) on Extinguishment of Debt		10,864,000
Fund II
Equity method investment
Gains (Losses) on Extinguishment of Debt		1,207,000
Milford Land [Member]
Combined summary of the operating results of the accounted for using the equity method
Casualty and Impairment Loss (Gain)			6,500,000
Avalon at Edgewater [Member]
Combined summary of the operating results of the accounted for using the equity method
Casualty and Impairment Loss (Gain)		(8,702,000)	793,000
Insurance Proceeds		29,008,000	22,142,000		73,008,000
Real Estate Investment Property, Net		21,844,000			$ 21,844,000
New England [Member]
Combined summary of the operating results of the accounted for using the equity method
Casualty and Impairment Loss (Gain)			$ 4,195,000
New England [Member] | Avalon at Edgewater [Member]
Combined summary of the operating results of the accounted for using the equity method
Casualty and Impairment Loss (Gain)		$ 26,039,000
Notes Payable Maturities 2020 [Member] | Mortgage notes payable | Avalon Hoboken [Member]
Combined summary of the operating results of the accounted for using the equity method
Debt Instrument, Interest Rate, Effective Percentage						4.18%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	[1]					$ 67,904,000

[1] Balances at March 31, 2016 and December 31, 2015 exclude $16,652 and $19,686, respectively, of debt premium, and $14,760 and $14,703, respectively, of deferred financing costs, as reflected in mortgage notes payable on the Company’s Condensed Consolidated Balance Sheets.