Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Unsecured notes payable	$ 1,582,655	$ 1,824,930
Secured notes payable	897,971	899,757
Total notes payable	2,480,600	2,724,700
Commercial Banks [Member]
Unsecured notes payable	0	244,000
Senior Unsecured Notes [Member]
Unsecured notes payable	$ 1,582,700	$ 1,580,900
5.83% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	May 15, 2017
Notes payable, effective interest rate	5.83%	5.83%
Unsecured notes payable	$ 246,600	$ 246,300
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	$ 248,300	$ 248,000
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	$ 345,800	$ 345,400
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	$ 247,100	$ 246,800
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	$ 248,200	$ 248,000
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	$ 246,700	$ 246,400
Secured Notes 1.27% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable	$ 866,800	867,400
Secured Notes 2.28% Floating Rate Tax Exempt Mortgage Note Due 2028 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date	Jun. 15, 2028
Notes payable, effective interest rate	2.28%
Secured notes payable	$ 31,100	32,400
Secured Notes 1.27% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable	$ 175,000	175,000
Minimum [Member] | Secured Notes 1.27% 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	1.27%
Minimum [Member] | Secured Notes 1.27% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate	1.27%
Maximum [Member] | Secured Notes 1.27% 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	225,000
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 0	$ 19,000