Detail of Real Estate and Associated Funding Underlying the Company's Unconsolidated Investments (Detail) (USD $) In Thousands, unless otherwise specified					9 Months Ended										9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Unconsolidated Real Estate Investments Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Other Development Joint Ventures Aria at Hathorne - Danvers, MA Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Other Operating Joint Ventures Avalon Chrystie Place I - New York, NY Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Other Operating Joint Ventures Avalon at Mission Bay North II - San Francisco, CA Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Other Operating Joint Ventures Avalon Del Rey - Los Angeles, CA Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Total Other Joint Ventures Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Lakeside - Chicago, IL Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Columbia - Baltimore, MD Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Sunset - Los Angeles, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon at Poplar Creek - Chicago, IL Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon at Civic Center - Norwalk, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Paseo Place - Fremont, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon at Yerba Buena - San Francisco, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon at Aberdeen Station - Aberdeen, NJ Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments The Springs - Corona, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Lombard - Lombard, IL Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Cedar Place - Columbia, MD Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Centerpoint - Baltimore, MD Fund I MortgageLoan Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Middlesex Crossing - Billerica, MA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Crystal Hill - Ponoma, NY Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Skyway - San Jose, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Rutherford Station - East Rutherford, NJ Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments South Hills Apartments - West Covina, CA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Weymouth Place - Weymouth, MA Fund I Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Bellevue Park - Bellevue, WA Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Fair Oaks - Fairfax, VA Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Avalon Rothbury - Gaithersburg, MD Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments The Apartments at Briarwood - Owings Mills, MD Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Grove Park Apartments - Gaithersburg, MD Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Creekside Meadows - Tustin, CA Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Canyonwoods - Lake Forest, CA Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Fox Run Apartments - Plainsboro, NJ Fund II MortgageLoan Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Waterstone Carlsbad - Carlsbad, CA Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Yale Village - Rockville, MD Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Fund II Home		Sep. 30, 2011 Unconsolidated Real Estate Investments Fund II Captain Parker Arms - Lexington, MA Home		Jul. 31, 2011 Unconsolidated Real Estate Investments Fund II Captain Parker Arms - Lexington, MA Home	Sep. 30, 2011 Unconsolidated Real Estate Investments Fund I Home
Investment Holdings [Line Items]
Company Ownership Percentage					50.00%	[1]	20.00%	[2]	25.00%	[2]	30.00%																																																												31.30%					15.20%
# of Apartment Homes			9,013		64	[1]	361	[2]	313	[2]	309		1,047		204		170		82		196		192		134		160		290		320	[3]	256		156		392	[4]	252		168		348		108		85		211		220		491		203		348		684		628		140		776	[5]	448		210		4,242		94		94	3,724
Total Capitalized Cost	$ 8,336,389	$ 8,084,574	$ 1,680,517	[6]			$ 136,572	[2],[6]	$ 124,060	[2],[6]	$ 70,080	[6]	$ 330,712	[6]	$ 18,581	[6]	$ 29,410	[6]	$ 20,903	[6]	$ 28,132	[6]	$ 42,756	[6]	$ 25,081	[6]	$ 66,811	[6]	$ 58,614	[6]	$ 29,926	[3],[6]	$ 35,323	[6]	$ 24,505	[6]	$ 80,257	[4],[6]	$ 38,406	[6]	$ 38,645	[6]	$ 78,250	[6]	$ 36,821	[6]	$ 24,756	[6]	$ 25,298	[6]	$ 33,993	[6]	$ 72,164	[6]	$ 31,481	[6]	$ 45,413	[6]	$ 102,028	[6]	$ 100,462	[6]	$ 25,459	[6]	$ 87,288	[5],[6]	$ 78,661	[6]	$ 49,529	[6]	$ 647,330	[6]	$ 20,852	[6]		$ 702,475	[6]
Debt Amount			$ 1,058,887	[7]	$ 1,860	[1],[7]	$ 117,000	[2],[7]	$ 105,000	[2],[7]	$ 44,464	[7]	$ 268,324	[7]	$ 12,056	[7]	$ 22,275	[7]	$ 12,750	[7]	$ 16,500	[7]	$ 27,001	[7]	$ 11,800	[7]	$ 41,500	[7]	$ 39,842	[7]	$ 23,653	[3],[7]	$ 17,243	[7]	$ 12,000	[7]	$ 45,000	[4],[7]	$ 24,100	[7]	$ 24,500	[7]	$ 37,500	[7]	$ 19,544	[7]	$ 11,761	[7]	$ 13,455	[7]	$ 21,515	[7]	$ 42,600	[7]	$ 18,750	[7]	$ 26,850	[7]	$ 63,200	[7]	$ 59,100	[7]			$ 54,296	[5],[7]	$ 46,141	[7]	$ 32,131	[7]	$ 378,083	[7]	$ 13,500	[7]		$ 412,480	[7]
Debt Interest Rate			4.70%	[8]	7.98%	[1],[8]	0.88%	[2],[8]	6.02%	[2],[8]	3.53%	[8]	3.40%	[8]	5.74%	[8]	5.48%	[8]	5.41%	[8]	4.83%	[8]	5.38%	[8]	5.74%	[8]	5.88%	[8]	5.64%	[8]	6.06%	[3],[8]	5.43%	[8]	5.68%	[8]	5.74%	[4],[8]	5.49%	[8]	5.43%	[8]	6.11%	[8]	6.13%	[8]	5.92%	[8]	5.12%	[8]	5.52%	[8]	5.26%	[8]	2.78%	[8]	3.64%	[8]	5.42%	[8]	3.81%	[8]			4.56%	[5],[8]	4.68%	[8]	4.26%	[8]	4.50%	[8]	3.90%	[8]		5.70%	[8]
Debt Type					Variable	[1]	Variable	[2]	Fixed	[2]	Variable				Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Fixed	[3]	Fixed		Fixed		Fixed	[4]	Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Fixed		Variable		Fixed		Fixed		Fixed		N/A		Fixed	[5]	Fixed		Fixed		-		Fixed
Debt Maturity Date					2010-06	[1]	2036-11	[2]	2015-12	[2]	2016-04				2012-03		2013-04		2014-03		2013-10		2013-08		2014-11		2014-03		2013-09		2014-10	[3]	2014-01		2015-02		2014-12	[4]	2014-12		2014-12		2014-03		2016-09		2014-10		2015-03		2019-06		2017-05		2017-06		2017-11		2018-01		2017-10				2014-11	[5]	2018-02		2019-08				2019-09

[1] As of September 30, 2011, the amounts under this borrowing have not been repaid and the lender has declared an event of default with respect to the note and required the venture to pay a default rate of interest, which is not material to the Company. Although the Company has not guaranteed the debt of Aria at Hathorne nor does it have any obligation to fund the debt should the venture be unable to do so, the Company has the right to cure any event of default by the venture. The Company determined that the value of its investment was impaired and this impairment was other than temporary, recognizing a charge in the third quarter of 2011.

[2] After the venture makes certain threshold distributions to the third-party partner, the Company generally receives 50% of all further distributions.

[3] Beginning in the third quarter of 2010, the Company consolidated the net assets and results of operations of The Springs.

[4] Borrowing on this community is comprised of three mortgage loans.

[5] Borrowing on this community is comprised of two mortgage loans.

[6] Represents total capitalized cost as of September 30, 2011.

[7] The Company has not guaranteed the debt of its unconsolidated investees and bears no responsibility for the repayment, other than the construction and completion and related financing guarantee for Avalon Chrystie Place I associated with the construction completion and occupancy certificate.

[8] Represents weighted average rate on outstanding debt as of September 30, 2011.