Impairment Charges (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Impairment charges on consolidated assets and unconsolidated joint venture investments
Undeveloped land		$ 3,800,000
Assets marketed for sale	13,500,000
Total continuing operations	13,517,000	3,856,000
Sold assets or assets held for sale	3,800,000	2,000,000
Total discontinued operations	3,854,000	1,983,000
Impairment of joint venture investments	(560,000)	(35,000)
Total impairment charges	$ 17,900,000	$ 5,800,000