Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Unsecured notes payable		$ 3,225,799	$ 2,524,099
Total notes payable (2)		$ 3,225,800	$ 2,524,100
Commercial Banks [Member] | 1.17% Term loan, due 2022 [Member]
Debt Instrument, Maturity Date		Jan. 01, 2022
Notes payable, effective interest rate		1.15%	2.70%
Unsecured notes payable		$ 99,800	$ 99,700
Commercial Banks [Member] | Unsecured Floating Rate Debt [Member]
Notes payable, effective interest rate		0.00%
Senior Unsecured Notes [Member]
Unsecured notes payable	[1]	$ 3,126,000	$ 2,380,400
Senior Unsecured Notes [Member] | 3.15% Notes Due 2022 [Member]
Debt Instrument, Maturity Date		Dec. 15, 2022
Notes payable, effective interest rate		3.15%	3.15%
Unsecured notes payable		$ 348,500	$ 348,000
Senior Unsecured Notes [Member] | 5.07% Notes Due 2023 [Member]
Debt Instrument, Maturity Date		Jun. 15, 2023
Notes payable, effective interest rate		5.07%	5.07%
Unsecured notes payable		$ 248,800	$ 248,400
Senior Unsecured Notes [Member] | 4.36% Notes Due 2024 [Member]
Debt Instrument, Maturity Date		Jan. 15, 2024
Notes payable, effective interest rate		4.36%	4.36%
Unsecured notes payable		$ 249,200	$ 249,000
Senior Unsecured Notes [Member] | 3.68% Notes Due 2024 [Member]
Debt Instrument, Maturity Date		Sep. 15, 2024
Notes payable, effective interest rate		3.68%	3.68%
Unsecured notes payable		$ 248,300	$ 248,000
Senior Unsecured Notes [Member] | 3.74% Notes Due 2028 [Member]
Debt Instrument, Maturity Date		Oct. 15, 2028
Notes payable, effective interest rate		3.74%	3.74%
Unsecured notes payable		$ 397,100	$ 396,700
Senior Unsecured Notes [Member] | 3.67% Notes Due 2029 [Member]
Debt Instrument, Maturity Date		Jul. 01, 2029
Notes payable, effective interest rate		3.67%	3.67%
Unsecured notes payable		$ 594,100	$ 593,700
Senior Unsecured Notes [Member] | 2.91% Notes, due 2030
Debt Instrument, Maturity Date		May 15, 2030
Notes payable, effective interest rate		2.91%	0.00%
Unsecured notes payable		$ 743,400	$ 0
Senior Unsecured Notes [Member] | 3.41% Notes Due 2049 [Member]
Debt Instrument, Maturity Date		Nov. 01, 2049
Notes payable, effective interest rate		3.41%	3.41%
Unsecured notes payable		$ 296,600	$ 296,600
Unsecured Credit Facility [Member] | Commercial Banks [Member]
Unsecured notes payable		0	44,000
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 99,800	$ 143,700

[1] Unamortized debt discounts and debt issuance costs of $24.2 million and $19.9 million are included in senior unsecured and secured notes payable as of September 30, 2020 and December 31, 2019, respectively.