Debt Mortgage Notes Payable (Details) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Equity Method Investment, Ownership Percentage		20.00%
Debt Instrument, Face Amount			$ 1,000,000,000	$ 600,000,000
Mortgage notes payable		3,898,369,000	4,111,487,000
Proceeds from (Repayments of) Debt		364,300,000	991,700,000
Mortgage Loans on Real Estate, New Mortgage Loans		26,500,000	190,900,000
Assumed mortgage debt		137,600,000	158,200,000
Properties acquired	1	9	21	1
Prepayment Penalty Charges		300,000
Write-off of unamortized deferred financing costs		1,600,000	4,400,000
Various period of outstanding mortgage indebtedness		June 15, 2051	September 1, 2048
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum		0.11%	0.05%
Interest rate range to maximum		11.25%	11.25%
Weighted Average Mortgage Debt Interest Rate		4.96%	4.84%
Historical cost, net of accumulated depreciation, of encumbered properties		4,400,000,000	4,900,000,000
Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		500,000,000
Write-off of unamortized deferred financing costs		1,000,000
Credit enhanced debt [Member]
Debt Instrument [Line Items]
Mortgage notes payable		$ 362,200,000	$ 455,600,000
Properties Acquired with Debt [Member]
Debt Instrument [Line Items]
Properties acquired		2	5