Indebtedness (Unsecured Lines of Credit) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Line of credit balance	$ 48,000	$ 48,000	$ 71,000
Unsecured Line of Credit DRLP [Member]
Line of Credit Facility, Increase, Additional Borrowings		400,000
Line of Credit Facility, Maximum Borrowing Capacity	1,600,000	$ 1,600,000
Line of Credit Facility, Covenant Compliance		we were in compliance with all covenants under this line of credit.
Unsecured Line of Credit DRLP [Member] | Unsecured Line of Credit DRLP [Member]
Maximum Capacity	1,200,000	$ 1,200,000
Maturity date		Jan. 01, 2019
Line of credit balance	$ 48,000	$ 48,000
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	0.925%		1.05%
Line of Credit Facility, Interest Rate at Period End	1.70%	1.70%