Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Book value	$ 3,355,455	$ 3,385,051
Fair value	3,539,844	3,750,361
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	490,226
Payments/Payoffs	(516,329)
Adjustments to fair value	(184,414)
Long-term Debt, Gross	3,391,843	3,418,157
Debt Issuance Costs, Net	36,388	33,106
Fixed Rate Secured Debt [Member]
Book value	59,543	62,817
Fair value	61,409	65,848
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(3,063)
Adjustments to fair value	(1,376)
Variable Rate Secured Debt [Member]
Book value	1,300	1,600
Fair value	1,300	1,600
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(300)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,175,000	3,058,740
Fair value	3,321,135	3,387,913
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	490,226
Payments/Payoffs	(373,966)
Adjustments to fair value	(183,038)
Unsecured Line of Credit DRLP [Member]
Book value	156,000	295,000
Fair value	156,000	$ 295,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(139,000)
Adjustments to fair value	$ 0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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