Indebtedness (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Repayments of Secured Debt	$ 30,349,000	$ 102,869,000
Proceeds from unsecured debt issuance	500,000,000	300,000,000
Repayments of Unsecured Debt	425,967,000	908,000
Fixed Rate Secured Debt [Member]
Estimated rate of fixed rate secured debt, minimum	3.50%
Estimated rate of fixed rate secured debt, maximum	4.90%
number of secured loans repaid	3
Repayments of Secured Debt	22,700,000
Repaid Debt, Weighted Average Interest Rate	5.73%
Number Of Secured Loans Assumed In Property Acquisition	3
Debt Instrument, Face Amount	99,300,000
Long-term Debt, Fair Value	103,600,000
Assumed Debt-Weighted Average Remaining Term	1 year 9 months 9 days
Debt, Weighted Average Interest Rate	5.59%
Debt Instrument, Interest Rate, Effective Percentage	3.00%
Fixed Rate Unsecured Debt [Member]
Proceeds from unsecured debt issuance	250,000,000
Estimated trading value of fixed rate unsecured debt, minimum percentage	93.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage	125.00%
Debt Instrument, Interest Rate, Stated Percentage	3.625%
Debt Instrument, Interest Rate, Effective Percentage	3.72%
Debt Instrument, Covenant Compliance	We were in compliance with all such covenants as of June 30, 2013
number of unsecured notes repaid	2
Repayments of Unsecured Debt	425,000,000
Fixed Rate Unsecured Debt Repaid [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.68%
Debt Instrument, Interest Rate, Effective Percentage	6.40%
Variable Rate UnSecured Debt [Member]
Proceeds from unsecured debt issuance	$ 250,000,000
Debt Instrument, Basis Spread on Variable Rate	1.35%
Debt Instrument, Interest Rate at Period End	1.55%
Unsecured Line of Credit DRLP [Member]
Debt Instrument, Basis Spread on Variable Rate	1.25%