Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Unsecured notes payable		$ 2,079,136	$ 1,836,427
Secured notes payable		45,683	485,176
Total notes payable		2,124,800	2,321,600
Senior Unsecured Notes [Member]
Unsecured notes payable	[1]	1,737,500	1,736,800
Secured Debt [Member]
Secured notes payable		$ 45,700	485,200
Term loan, due 2022 [Member] | Commercial Banks [Member]
Debt Instrument, Maturity Date		Jan. 01, 2022
Unsecured notes payable		$ 99,600	$ 99,600
4.78% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jun. 15, 2021
Notes payable, effective interest rate		4.78%	4.78%
Unsecured notes payable		$ 249,200	$ 249,100
3.15% Notes Due 2022 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Dec. 15, 2022
Notes payable, effective interest rate		3.15%	3.15%
Unsecured notes payable		$ 347,400	$ 347,300
5.07% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jun. 15, 2023
Notes payable, effective interest rate		5.07%	5.07%
Unsecured notes payable		$ 248,100	$ 248,000
4.36% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Jan. 15, 2024
Notes payable, effective interest rate		4.36%	4.36%
Unsecured notes payable		$ 248,800	$ 248,700
3.68% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Sep. 15, 2024
Notes payable, effective interest rate		3.68%	3.68%
Unsecured notes payable		$ 247,700	$ 247,600
3.74% Notes Due 2028 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date		Oct. 15, 2028
Notes payable, effective interest rate		3.74%	3.74%
Unsecured notes payable		$ 396,300	$ 396,100
4.38% Conventional Mortgage Notes, due 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Sep. 01, 2045
Notes payable, effective interest rate		4.38%	4.38%
Secured notes payable		$ 45,700	$ 45,900
5.19% Conventional Mortgage Notes, due 2019 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		May 01, 2019
Notes payable, effective interest rate		0.00%	5.19%
Secured notes payable		$ 0	$ 419,900
5.33% Conventional Mortgage Notes Due 2019 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Jun. 01, 2019
Notes payable, effective interest rate		0.00%	5.33%
Secured notes payable		$ 0	$ 19,400
Unsecured 3.33% Floating Rate Debt [Member] | Commercial Banks [Member]
Notes payable, effective interest rate		3.33%
Unsecured notes payable		$ 341,600	99,600
Short-term Debt [Member]
Other Short-term Borrowings		0
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable		341,600	99,600
Unsecured Credit Facility [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 242,000	$ 0

[1] Unamortized debt discounts and debt issuance costs of $13.1 million and $13.9 million are included in senior unsecured and secured notes payable as of March 31, 2019 and December 31, 2018, respectively.