Real Estate Sales (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2011 Home	Dec. 31, 2010 Home	Dec. 31, 2009 Home	Sep. 30, 2011 Avalon Warner Park, Canoga Park, CA Home		Sep. 30, 2011 Juanita Village Phase 2, Kirkland, WA Home		Sep. 30, 2011 Avalon Danvers Lowlands, Danvers, MA Home		Dec. 31, 2011 Avalon at Rock Spring, Bethesda, MD Home	Dec. 31, 2011 Avalon at Cameron Court, Alexandria, VA Home	Dec. 31, 2011 Avalon at Stratford Green, Bloomingdale, IL Home
Real Estate Properties [Line Items]
Apartment homes	1,038	1,007	1,037		[1]		[1]		[1]	386	460	192
Debt
Gross sales price	292,965	198,600	179,675	23,000		9,850		1,625		73,750	146,240	38,500
Proceeds from sale of real estate	$ 287,358	$ 194,009	$ 176,481	$ 22,147		$ 9,411		$ 1,551		$ 72,370	$ 144,570	$ 37,309

[1] Disposition of an unimproved land parcel.