Indebtedness (Unsecured Lines of Credit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Line of credit balance	$ 71,000	$ 106,000
Unsecured Line of Credit DRLP [Member]
Line of credit balance			$ 285,000
Line of Credit Facility, Increase, Additional Borrowings	400,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,600,000
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%
Unsecured Line of Credit DRLP [Member] | Unsecured Line of Credit DRLP [Member]
Maximum Capacity	$ 1,200,000
Maturity date	Jan. 01, 2019
Line of credit balance	$ 71,000
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Line of Credit DRLP [Member]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.05%
Line of Credit Facility, Interest Rate at Period End	1.41%