Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Unsecured notes payable	$ 1,770,491	$ 1,837,911
Secured notes payable	904,196	905,628
Total notes payable	2,674,700	2,743,500
Commercial Banks [Member]
Unsecured notes payable	182,000	0
Senior Unsecured Notes [Member]
Unsecured notes payable	$ 1,588,500	$ 1,837,900
5.08% Notes Due 2015 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Jun. 15, 2015
Notes payable, effective interest rate	5.08%	5.08%
Unsecured notes payable	$ 0	$ 249,900
5.75% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	May 15, 2017
Notes payable, effective interest rate	5.75%	5.75%
Unsecured notes payable	$ 246,600	$ 246,500
4.70% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Jun. 15, 2021
Notes payable, effective interest rate	4.70%	4.70%
Unsecured notes payable	$ 249,000	$ 249,000
3.07% Notes Due 2022 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Dec. 15, 2022
Notes payable, effective interest rate	3.07%	3.07%
Unsecured notes payable	$ 347,100	$ 347,000
5.00% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Jun. 15, 2023
Notes payable, effective interest rate	5.00%	5.00%
Unsecured notes payable	$ 248,000	$ 247,800
4.27% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Jan. 15, 2024
Notes payable, effective interest rate	4.27%	4.27%
Unsecured notes payable	$ 249,600	$ 249,600
3.59% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Debt Instrument, Maturity Date	Sep. 15, 2024
Notes payable, effective interest rate	3.59%	3.59%
Unsecured notes payable	$ 248,200	$ 248,100
Secured Notes 0.94% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable	$ 870,300	870,900
Secured Notes 1.31% Floating Rate Tax Exempt Mortgage Note Due 2028 [Member] [Domain] | Secured Debt [Member]
Debt Instrument, Maturity Date	Jun. 15, 2028
Notes payable, effective interest rate	1.31%
Secured notes payable	$ 33,900	34,700
Secured Notes 0.94% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable	$ 175,000	175,000
Minimum [Member] | Secured Notes 0.94% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date	Oct. 01, 2018
Notes payable, effective interest rate	0.94%
Minimum [Member] | Secured Notes 0.94% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate	0.94%
Maximum [Member] | Secured Notes 0.94% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Debt Instrument, Maturity Date	Sep. 01, 2045
Notes payable, effective interest rate	5.63%
Line of Credit [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 175,000	0
Short-term Debt [Member]
Debt Instrument, Maturity Date	May 14, 2016
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 7,000	$ 0