Indebtedness (Unsecured Line of Credit) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Unsecured Line of Credit DRLP [Member]
Line of Credit Facility, Covenant Compliance	we were in compliance with all financial covenants under this line of credit.
Maximum Capacity	$ 1,200,000
Maturity date	Jan. 30, 2022
Line of Credit Facility Option to Increase Borrowing Limit	$ 800,000
Line of credit balance	0
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,000,000
Unsecured Line of Credit DRLP [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	0.875%