Indebtedness (Unsecured Lines of Credit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Document Period End Date	Dec. 31, 2019
Line of credit balance	$ 0	$ 30,000
Unsecured Line of Credit DRLP [Member]
Maximum Capacity	$ 1,200,000
Maturity date	Jan. 30, 2022
Line of credit balance	$ 0
Line of Credit Facility, Increase, Additional Borrowings	800,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,000,000
Line of Credit Facility, Covenant Compliance	we were in compliance with all financial covenants under this line of credit.
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Description of Variable Rate Basis	LIBOR
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Line of Credit DRLP [Member]
Debt Instrument, Basis Spread on Variable Rate	0.875%