Indebtedness (Unsecured Lines of Credit) (Details) (USD $)	12 Months Ended					1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Unsecured Line of Credit - DRLP [Member]	Dec. 31, 2012 Line of Credit [Member]	Jul. 31, 2012 Unsecured Line of Credit - Consolidated Subsidiary [Member]
Line of Credit Facility, Interest Rate at Period End				1.47%
Maximum Capacity				$ 850,000,000		$ 30,000,000
Maturity date				Dec. 01, 2015
Line of credit balance				285,000,000
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.
Current Market rate of Unsecured lines of Credit					1.61%
Proceeds from (Repayments of) Lines of Credit	$ 264,707,000	$ (172,753,000)	$ 177,276,000			$ 20,300,000