Indebtedness (Details) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Repayments of mortgage loans	$ 26,960,000	$ 26,976,000
Secured Debt	281,170,000		290,819,000
Proceeds from unsecured notes	400,000,000	0
Mortgage Loans on Real Estate [Member] | Issuance of Debt [Member]
Debt Instrument [Line Items]
Interest rate on debt	4.84%
Proceeds from Issuance of Secured Debt	45,000,000
Secured Debt	17,300,000
Mortgage Loans on Real Estate [Member] | Repayment of Debt [Member]
Debt Instrument [Line Items]
Repayments of mortgage loans	24,400,000
Interest rate on debt	7.47%
Unsecured Debt [Member] | Issuance of Debt [Member]
Debt Instrument [Line Items]
Proceeds from unsecured notes	$ 400,000,000
Debt Instrument, Maturity Date, Description	10-year
Debt Instrument, Interest Rate, Stated Percentage	4.125%