Indebtedness (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Repayments of Secured Debt	$ 17,486,000	$ 10,455,000
Proceeds from unsecured debt issuance	250,000,000	0
Fixed Rate Secured Debt [Member]
Estimated rate of fixed rate secured debt, minimum	2.80%
Estimated rate of fixed rate secured debt, maximum	4.30%
number of secured loans repaid	2
Repayments of Secured Debt	13,700,000
Repaid Debt, Weighted Average Interest Rate	6.20%
Fixed Rate Unsecured Debt [Member]
Estimated trading value of fixed rate unsecured debt, minimum percentage	100.00%
Estimated trading value of fixed rate unsecured debt, maximum percentage	130.00%
Proceeds from unsecured debt issuance	250,000,000
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 March 31, 2013
Unsecured Line of Credit DRLP [Member]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Line of Credit Facility, Increase, Additional Borrowings	400,000,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,250,000,000
Line of Credit Facility, Covenant Compliance	we were in compliance with all covenants under this line of credit.