Notes Payable (Summary Of Indebtedness) (Details) - USD ($) $ in Thousands		9 Months Ended
	Jun. 15, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Unsecured notes payable		$ 1,811,739		$ 1,837,911
Secured notes payable		903,468		905,628
Total notes payable		2,715,200		2,743,500
Commercial Banks [Member]
Unsecured notes payable		223,000		0
Senior Unsecured Notes [Member]
Unsecured notes payable		1,588,700		$ 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		$ 249,100		$ 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		$ 347,200		$ 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 0.95% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] | Secured Debt [Member]
Secured notes payable		$ 870,000		870,900
Secured Notes 1.24% 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		1.24%
Secured notes payable		$ 33,500		34,700
Secured Notes 0.95% Floating Rate [Member] | Secured Debt [Member]
Secured notes payable		$ 175,000		175,000
Minimum [Member] | Secured Notes 0.95% 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		0.95%
Minimum [Member] | Secured Notes 0.95% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate		0.95%
Maximum [Member] | Secured Notes 0.95% 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%
Unsecured Credit Facility [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 200,000		0
Short-term Debt [Member]
Debt Instrument, Maturity Date		May 14, 2016
Short-term Debt [Member] | Commercial Banks [Member]
Unsecured notes payable		$ 23,000		$ 0