Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Book value	$ 3,075,186	$ 2,914,765
Fair value	3,391,058	3,081,932
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	693,400
Payments/Payoffs	(518,166)
Adjustments to fair value	133,892
Long-term Debt, Gross	3,109,377	2,934,187
Debt Issuance Costs, Net	34,191	19,422
Fixed Rate Secured Debt [Member]
Book value	48,737	32,287
Fair value	41,591	34,547
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	18,400
Payments/Payoffs	(1,906)
Adjustments to fair value	(9,450)
Variable Rate Secured Debt [Member]
Book value	1,900	1,900
Fair value	1,900	1,900
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,058,740	2,900,000
Fair value	3,347,567	$ 3,045,485
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	675,000
Payments/Payoffs	(516,260)
Adjustments to fair value	$ 143,342

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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