Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Book value	$ 3,080,262	$ 2,908,477
Fair value	3,199,313	3,034,065
Issuances and Assumptions	189,000
Payments/Payoffs	(18,155)
Adjustments to fair value	(5,597)
Long-term Debt, Gross	3,102,326	2,931,529
Deferred Finance Costs, Net	(22,064)	(23,052)
Fixed Rate Secured Debt [Member]
Book value	364,306	381,894
Fair value	392,885	415,231
Issuances and Assumptions	0
Payments/Payoffs	(17,539)
Adjustments to fair value	(4,807)
Variable Rate Secured Debt [Member]
Book value	2,800	2,800
Fair value	2,800	2,800
Issuances and Assumptions	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,498,220	2,498,835
Fair value	2,566,628	2,568,034
Issuances and Assumptions	0
Payments/Payoffs	(616)
Adjustments to fair value	(790)
Unsecured Line of Credit DRLP [Member]
Book value	237,000	48,000
Fair value	237,000	$ 48,000
Issuances and Assumptions	189,000
Payments/Payoffs	0
Adjustments to fair value	$ 0

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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