Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Unsecured notes payable	$ 1,866,502	$ 1,824,930
Secured notes payable	899,315	899,757
Total notes payable	2,765,800	2,724,700
Commercial Banks [Member]
Unsecured notes payable	285,000	244,000
Senior Unsecured Notes [Member]
Unsecured notes payable	$ 1,581,500	$ 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	$ 246,400	$ 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	$ 248,200	$ 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	$ 345,500	$ 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	$ 246,900	$ 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	$ 248,000	$ 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	$ 246,500	$ 246,400
Secured Notes 1.38% to 5.77% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable	$ 867,200	867,400
Secured Notes 1.62% Floating Rate Tax Exempt Mortgage Note Due 2028 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date	Jun. 15, 2028
Notes payable, effective interest rate	1.62%
Secured notes payable	$ 32,100	32,400
Secured Notes 1.38% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable	$ 175,000	175,000
Minimum [Member] | Secured Notes 1.38% 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	1.38%
Minimum [Member] | Secured Notes 1.38% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate	1.38%
Maximum [Member] | Secured Notes 1.38% 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	$ 265,000	225,000
Short-term Debt [Member]
Debt Instrument, Maturity Date	May 14, 2016
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 20,000	$ 19,000