Borrowing Arrangements (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Debt assumed in connection with acquisition	$ 412,825,000	$ 3,651,089,000	$ 125,320,000
Gain on extinguishment of debt	(37,640,000)	(27,604,000)	(9,791,000)
Mortgage loans
Debt Instrument [Line Items]
Number of mortgage loans outstanding	244
Mortgage loans outstanding balance	2,900,000,000
Number of properties which are held as collateral for the mortgage loans	248
Number of mortgage loans bearing interest at fixed rates	223
Principal amount outstanding of variable rate mortgage loans	2,500,000,000
Number of mortgage loans bearing interest at lender's variable rates	21
Principal amount outstanding of variable rate mortgage loans	438,000,000
Weighted average annual rate on fixed rate mortgage loans (as a percent)	6.10%
Weighted average annual rate on variable rate mortgage loans (as a percent)	1.90%
Weighted average maturity period of mortgage loans (in years)	5 years 7 months 6 days
Debt assumed in connection with acquisition	380,300,000
Mortgage loans repaid in full	344,200,000
Gain on extinguishment of debt	$ 2,100,000
Mortgage loans | Minimum
Debt Instrument [Line Items]
Fixed interest rate (as a percent)	3.99%
Interest rate on mortgage loans bearing interest at lender's variable rates (as a percent)	1.02%
Mortgage loans | Maximum
Debt Instrument [Line Items]
Fixed interest rate (as a percent)	8.63%
Interest rate on mortgage loans bearing interest at lender's variable rates (as a percent)	7.30%