Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Book value	$ 3,381,060	$ 3,320,141
Fair value	3,572,538	3,487,842
Issuances and Assumptions	77,000
Payments/Payoffs	(16,956)
Adjustments to fair value	24,652
Long-term Debt, Gross	3,401,628	3,341,739
Deferred Finance Costs, Net	(20,568)	(21,598)
Fixed Rate Secured Debt [Member]
Book value	720,363	736,896
Fair value	775,449	789,095
Issuances and Assumptions	0
Payments/Payoffs	(16,377)
Adjustments to fair value	2,731
Variable Rate Secured Debt [Member]
Book value	3,100	3,100
Fair value	3,100	3,100
Issuances and Assumptions	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,530,165	2,530,743
Fair value	2,645,989	2,624,795
Issuances and Assumptions	0
Payments/Payoffs	(579)
Adjustments to fair value	21,773
Unsecured Line of Credit DRLP [Member]
Book value	148,000	71,000
Fair value	148,000	$ 70,852
Issuances and Assumptions	77,000
Payments/Payoffs	0
Adjustments to fair value	$ 148

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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