Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Book value	$ 2,914,765	$ 2,658,501
Long-term Debt, Gross	2,934,187	2,684,801
Debt Issuance Costs, Net	19,422	26,300
Fair value	3,081,932	$ 2,662,401
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	575,000
Payoffs	(325,515)
Adjustments to fair value	$ 170,046
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	5.92%	6.91%
Book value	$ 32,287	$ 77,601
Fair value	34,547	$ 80,238
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(45,215)
Adjustments to fair value	$ (476)
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	1.39%	1.72%
Book value	$ 1,900	$ 2,200
Fair value	1,900	$ 2,200
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(300)
Adjustments to fair value	$ 0
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	3.71%	3.92%
Book value	$ 2,900,000	$ 2,575,000
Fair value	3,045,485	$ 2,549,963
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	575,000
Payoffs	(250,000)
Adjustments to fair value	$ 170,522
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate	0.00%	3.39%
Book value	$ 0	$ 30,000
Fair value	0	$ 30,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(30,000)
Adjustments to fair value	$ 0