Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Book value	$ 2,422,891	$ 2,908,477
Fair value	2,518,801	3,034,065
Debt Issuance Costs, Net	21,114	23,052
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	300,000
Long-term Debt, Gross	2,444,005	$ 2,931,529
Payoffs	(787,442)
Adjustments to fair value	$ (27,822)
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	7.43%	7.13%
Book value	$ 309,463	$ 381,894
Fair value	325,753	$ 415,231
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(72,348)
Adjustments to fair value	$ (17,130)
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	1.85%	0.79%
Book value	$ 2,500	$ 2,800
Fair value	2,500	$ 2,800
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	3.89%	4.12%
Book value	$ 2,132,042	$ 2,498,835
Fair value	2,190,548	$ 2,568,034
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	300,000
Payoffs	(666,794)
Adjustments to fair value	(10,692)
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate		1.70%
Book value	0	$ 48,000
Fair value	0	$ 48,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payoffs	(48,000)
Adjustments to fair value	$ 0