Real Estate Activities (Narrative) (Details) (USD $)	12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 Land Parcels Sold [Member]	Dec. 31, 2009 Apartment Communities Sold [Member]	Dec. 31, 2011 Austin Condominium Project [Member]	Dec. 31, 2011 Atlanta Condominium Project [Member]	Jun. 30, 2010 Condominium Conversion Communities [Member]	Dec. 31, 2011 Condominium Communities [Member]	Dec. 31, 2010 Condominium Communities [Member]	Dec. 31, 2009 Condominium Communities [Member]	Dec. 31, 2011 Post Katy Trail [Member] Acquisition [Member]
Real Estate Properties [Line Items]
Number of real estate properties	58			2	3			2
Aggregate purchase price												$ 48,500
Square footage of real estate property												9,080
Purchase price allocated to land												7,324
Purchase price allocated to building, improvements and equipment												39,531
Purchase price allocated to lease related intangible assets												764
Purchase price allocated to other assets												881
Proceeds from sale of land		8,888,000
Gains on sales of real estate assets			79,366,000
Proceeds from sales of real estate assets	59,469,000	77,388,000	170,777,000		148,553,000
Number of units in real estate property	21,658				1,328	148	129					227
Condominiums, for-sale and under construction	54,845,000	82,259,000				35,539	19,306
Condominium homes sold	58	66	103
Net gains on condominium sales activities	$ 10,514,000	$ 6,161,000	$ 3,481,000						$ 10,514,000	$ 6,161,000	$ 3,481,000