Indebtedness (Unsecured Line of Credit) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Line of credit balance	$ 0	$ 106,000
Unsecured Line of Credit DRLP [Member]
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.
Line of Credit Facility Option to Increase Borrowing Limit	$ 400,000
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	$ 0
Unsecured Line of Credit DRLP [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.05%