Real Estate Disposition Activities - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2011 Property Community Home Location Investment	Sep. 30, 2011 Community Home Location Investment	Sep. 30, 2010	Dec. 31, 2008
Number of unimproved land parcels	3
Gain on sale of land	$ 13,716,000	$ 13,716,000
Proceeds from exchange/sale of real estate, net of selling costs	34,475,000	55,479,000	186,058,000
Impairment loss - land holdings	14,052,000	14,052,000		9,952,000
Number of operating apartment communities owned	184	184
Discontinued Operation
Number of operating apartment communities owned	1	1
Prior fiscal year
Impairment loss - land holdings	$ 20,200,000