Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Unsecured notes payable		$ 1,339,142	$ 1,338,628
Secured notes payable		865,798	865,970
Total notes payable		2,204,900	2,204,600
Senior Unsecured Notes [Member]
Unsecured notes payable	[1]	$ 1,339,100	$ 1,338,600
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		$ 248,800	$ 248,700
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		$ 346,800	$ 346,600
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		$ 247,700	$ 247,600
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		$ 248,500	$ 248,500
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		$ 247,300	$ 247,200
Secured Notes 2.20% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable		865,800	866,000
Secured Notes 2.20% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable		$ 175,000	$ 175,000
Minimum [Member] | Secured Notes 2.20% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Oct. 01, 2018
Notes payable, effective interest rate		2.20%
Minimum [Member] | Secured Notes 2.20% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate		2.20%
Maximum [Member] | Secured Notes 2.20% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date		Sep. 01, 2045
Notes payable, effective interest rate		5.77%
Unsecured Credit Facility [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 0
Short-term Debt [Member]
Debt Instrument, Maturity Date		May 14, 2018
Other Short-term Borrowings		$ 0

[1] Unamortized debt discounts and debt issuance costs of $11.6 million and $12.3 million are included in senior unsecured and secured notes payable as of March 31, 2018 and December 31, 2017, respectively.