Significant Accounting Policies Properties in Development (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property, Plant and Equipment [Line Items]
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	$ (896,125,000)
Other Cost and Expense, Operating	241,000	520,000	3,800,000
Deferred Costs, Leasing, Net	56,500,000	52,900,000
Impairment of Real Estate	15,900,000	23,900,000	103,900,000
Development in Process	224,077,000	610,932,000
Impairment Losses Related to Real Estate Partnerships	(5,880,000)	(8,750,000)	0
Deferred Finance Costs, Net	13,700,000	10,200,000
Properties in Development [Member]
Property, Plant and Equipment [Line Items]
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	2,100,000	899,000
Refundable deposits - development [Member]
Property, Plant and Equipment [Line Items]
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	1,000,000	840,000
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Development in Process	50,903,000	41,611,000
Construction Complete and in Lease-up [Member]
Property, Plant and Equipment [Line Items]
Development in Process	76,301,000	459,231,000
Land Held for Future Development [Member]
Property, Plant and Equipment [Line Items]
Development in Process	96,873,000	110,090,000
Segment, Discontinued Operations [Member]
Property, Plant and Equipment [Line Items]
Impairment of Real Estate	2,100,000	0	6,900,000
Subsidiaries [Member]
Property, Plant and Equipment [Line Items]
Impairment Losses Related to Real Estate Partnerships	$ 4,600,000	$ 2,700,000