Investments in Real Estate Entities (Details) (USD $)	12 Months Ended															1 Months Ended	12 Months Ended				12 Months Ended			12 Months Ended
	Dec. 31, 2013 home	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Fund I community investor loan item	Dec. 31, 2013 Fund I Avalon Yerba Buena	Dec. 31, 2013 Fund I Avalon at Civic Center	Dec. 31, 2013 Fund I Avalon at Cedar Place	Dec. 31, 2013 Fund I Avalon Sunset Towers	Dec. 31, 2013 Fund I Avalon Crystal Hill	Dec. 31, 2013 Fund I Avalon Centerpoint	Dec. 31, 2013 Fund I Middlesex Crossing	Dec. 31, 2013 Fund II loan investor	Dec. 31, 2013 Fund II Avalon Rothbury	Feb. 29, 2004 CVP I, LLC item	Dec. 31, 2004 MVP I, LLC item	Dec. 31, 2011 Aria at Hathorne Hill, LLC	Dec. 31, 2013 Aria at Hathorne Hill, LLC	Jun. 30, 2007 Aria at Hathorne Hill, LLC item	Dec. 31, 2013 Variable rate bonds maturing in November 2036 CVP I, LLC	Dec. 31, 2013 6.02% fixed-rate mortgage note MVP I, LLC	Dec. 31, 2013 Arna Valley View LP home	Dec. 31, 2013 Arna Valley View LP 6.25% fixed-rate tax-exempt bonds	Dec. 31, 2013 Arna Valley View LP 4% fixed-rate county bonds	Dec. 31, 2013 Arboretum at Burlington home
Investment in Real Estate Entities
Number of extension options				2
Period of extension				1 year
Number of institutional investors				9								6
Combined general partner and limited partner equity interest (as a percent)				15.20%								31.30%		20.00%	25.00%			50.00%			0.01%
Equity investment, net of distributions and excluding the purchase of mortgage note secured				$ 14,209,000								$ 93,714,000
Number of operating apartment communities sold				7
Proceeds from sale of real estate	919,442,000	274,018,000	287,358,000		103,000,000	45,844,000	26,000,000	25,800,000	33,050,000	75,250,000	40,250,000		39,600,000
Gain on sale of communities	96,152,000	106,195,000	22,246,000	11,484,000									2,790,000				975,000
Number of loans secured by individual assets				4								13
Debt outstanding				66,132,000								467,959,000							117,000,000	105,000,000		4,829,000	5,836,000
Percentage of pro rate share of amount paid related to guaranteed obligations will reimburse by partner																			80.00%
Maximum amount that will be paid for capital contributions made by partners (as a percent)				10.00%								10.00%
Maximum amount that will be paid for capital contributions made by partners				7,500,000								8,910,000
Number of apartment homes acquired																								312
Number of apartment homes														361	313			64			101
Purchase price																								79,850,000
Distributions percentage after achievement of a threshold return														50.00%	45.00%
Stated interest rate (as a percent)																				6.02%		6.25%
Percentage of apartment homes for which affordable rents are available with community																					100.00%
Debt interest rate																							4.00%
Percentage of homes built and sold																		30.00%
Gain on acquisition of unconsolidated entity		14,194,000
Impairment of investment in unconsolidated entities	0	0	1,955,000
Acquisition of mortgage note																$ 1,700,000
Number of apartment homes included in operating apartment communities owned	72,811