Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt, Weighted Average Interest Rate		4.54%
Book value	$ 3,385,051		$ 2,914,765
Long-term Debt, Gross	3,418,157		2,934,187
Debt Issuance Costs, Net	33,106		19,422
Fair value	3,750,361		$ 3,081,932
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	1,013,855
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(529,716)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 184,290
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	4.56%		5.92%
Book value	$ 62,817		$ 32,287
Fair value	65,848		$ 34,547
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	43,855
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(13,156)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 602
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	0.08%		1.39%
Book value	$ 1,600		$ 1,900
Fair value	1,600		$ 1,900
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(300)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 0
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	3.35%		3.71%
Book value	$ 3,058,740		$ 2,900,000
Fair value	3,387,913		$ 3,045,485
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	675,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(516,260)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 183,688
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate	1.03%		0.00%
Book value	$ 295,000		$ 0
Fair value	295,000		$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	295,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	$ 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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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