Notes Payable (Narrative) (Details) $ in Thousands		9 Months Ended
		Sep. 30, 2021 USD ($) yr	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Notes Payable	[1]	$ 3,169,400	$ 3,166,600
Unamortized debt discounts and debt issuance costs		20,600	23,400
Unsecured Debt		3,169,428	3,166,625
Available amount under unsecured credit facility		$ 885,200
Weighted Average Interest Rate	[2],[3]	3.60%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years) | yr		7.7
Letter Of Credit [Member]
Maximum Ability to Issue Letters of Credit Under Unsecured Credit Facility		$ 50,000
Outstanding balance under credit facility		14,800
Senior Unsecured Notes [Member]
Unsecured Debt		3,129,500	3,126,900
Unsecured Credit Facility [Member]
Maximum borrowing capacity under unsecured credit facility		$ 900,000
Maximum term of bid rate loans (days)		180 days
Lesser of amount stated or the amount available under the unsecured credit facility		$ 450,000
Floating rate notes payable [Member]
Notes Payable		$ 39,900	39,700	$ 99,800
Weighted Average Interest Rate		1.90%		1.20%
1.85% Term loan, due 2022 [Member] | Commercial Banks [Member]
Unsecured Debt		$ 39,900	$ 39,700
Notes payable, effective interest rate		1.85%	1.85%
Debt Instrument, Maturity Date		Jan. 01, 2022

[1] Unamortized debt discounts and debt issuance costs of $20.6 million and $23.4 million are included in senior unsecured notes payable as of September 30, 2021 and December 31, 2020, respectively.

[2] Includes all available extension options.

[3] Includes the effects of the applicable settled forward interest rate swaps.