Fair Value Measurements (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Assets:
Real estate	$ 2,728,883,000		$ 2,728,883,000		$ 2,979,937,000
Total losses pertaining to real estate	8,301,000	41,267,000	24,487,000	47,794,000
Investments in unconsolidated entities	65,919,000		65,919,000		71,280,000
Impairment losses of discontinued operations		40,721,000	599,000	41,192,000	38,800,000
Recurring basis | Total Fair Value
Assets:
Derivative financial instruments					31,000
Liabilities:
Derivative financial instruments	(1,604,000)		(1,604,000)		(621,000)
Recurring basis | Level 2
Assets:
Derivative financial instruments					31,000
Liabilities:
Derivative financial instruments	(1,604,000)		(1,604,000)		(621,000)
Nonrecurring basis
Assets:
Total losses pertaining to real estate			(24,487,000)		(53,970,000)
Total losses pertaining to investments in unconsolidated entities					(276,000)
Impairment losses for properties disposed					2,100,000
Nonrecurring basis | Total Fair Value
Assets:
Real estate	40,154,000		40,154,000		178,500,000
Investments in unconsolidated entities					6,304,000
Nonrecurring basis | Level 2
Assets:
Real estate					31,900,000
Nonrecurring basis | Level 3
Assets:
Real estate	40,154,000		40,154,000		146,600,000
Investments in unconsolidated entities					$ 6,304,000