Investments in Real Estate Entities (Details 2) - Subsequent Event Type [Domain]	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2015 USD ($) community	Jun. 30, 2015 USD ($) community home building	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) community Claim home plaintiff building	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Equity method investment
Long-term Debt		$ 6,433,889,000		$ 6,433,889,000		$ 6,448,138,000
Real Estate Investment Property, Net		13,495,510,000		13,495,510,000		13,219,140,000
Impairment of Real Estate		0	$ 0		$ 0
Casualty and Impairment (Gain) Loss		(17,114,000)	0	(11,326,000)	0
Assets:
Real estate, net		1,513,850,000		1,513,850,000		1,617,627,000
Other assets		60,793,000		60,793,000		72,290,000
Total assets		1,574,643,000		1,574,643,000		1,689,917,000
Liabilities and partners' capital:
Mortgage notes payable and credit facility		964,246,000		964,246,000		980,128,000
Other liabilities		21,232,000		21,232,000		24,884,000
Partners' capital		589,165,000		589,165,000		684,905,000
Total liabilities and partners' capital		1,574,643,000		1,574,643,000		1,689,917,000
Combined summary of the operating results of the accounted for using the equity method
Rental and other income		43,395,000	52,270,000	88,650,000	104,646,000
Operating and other expenses		(17,375,000)	(20,483,000)	(34,712,000)	(41,691,000)
Gain on sale of communities		0	5,682,000	32,490,000	5,682,000
Interest expense, net		(10,334,000)	(13,523,000)	(20,811,000)	(27,413,000)
Depreciation expense		(11,942,000)	(13,863,000)	(23,845,000)	(28,280,000)
Net income		3,744,000	10,083,000	41,772,000	12,944,000
Impairment of investment in unconsolidated entities		0	0	0	0
Expensed Acquisition, Development And Other Pursuit Costs, Net of Recoveries		$ 673,000	$ 2,017,000	$ 1,860,000	$ 2,732,000
Number of Real Estate Properties | community		257		257
Number of apartment homes included in operating apartment communities owned | home		74,857		74,857
MVPI LLC [Member]
Equity method investment
Long-term Debt		$ 103,000,000		$ 103,000,000
Fixed rate (as a percent)		3.24%		3.24%
Fund I and II
Combined summary of the operating results of the accounted for using the equity method
Costs in excess of equity in underlying net assets of the respective investments		$ 3,312,000		$ 3,312,000		$ 3,880,000
Fund II
Combined summary of the operating results of the accounted for using the equity method
Maximum amount that will be paid for capital contributions made by partners (as a percent)				10.00%
Maximum amount that will be paid for capital contributions made by partners		8,910,000		$ 8,910,000
Avalon Mills [Member]
Equity method investment
Impairment of Real Estate				800,000
Avalon at Edgewater [Member]
Equity method investment
Casualty Loss		4,886,000		5,977,000
Insurance Proceeds		22,000,000		44,142,000
Real Estate Investment Property, Net		21,844,000		21,844,000
Casualty and Impairment (Gain) Loss		$ (17,114,000)		$ (16,321,000)
Combined summary of the operating results of the accounted for using the equity method
Number of Real Estate Properties | building		2		2
Number of apartment homes included in operating apartment communities owned | home		168		168
Loss Contingency, New Claims Filed, Number | Claim				15
Loss Contingency, Number of Plaintiffs | plaintiff				120
New England [Member]
Equity method investment
Impairment of Real Estate				$ 4,195,000
Mortgage notes payable | MVPI LLC [Member]
Combined summary of the operating results of the accounted for using the equity method
Repayments of secured debt		$ 105,000,000
Notes payable maturing in 2015 | Mortgage notes payable
Equity method investment
Fixed rate (as a percent)	3.12%
Combined summary of the operating results of the accounted for using the equity method
Number of Real Estate Properties | community	8
Repayments of secured debt	$ 481,582,000