Notes Payable (Summary Of Indebtedness) (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Unsecured notes payable	$ 1,721,998,000	$ 1,538,212,000
Total secured notes payable	943,039,000	972,256,000
Total notes payable	2,665,000,000	2,510,500,000
Senior Unsecured Notes [Member]
Unsecured notes payable	1,539,000,000	1,538,200,000
5.45% Notes Due 2013 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	200,000,000	199,900,000
Notes payable, effective interest rate	5.45%
Notes payable, maturity date	2013
5.08% Notes Due 2015 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	249,700,000	249,500,000
Notes payable, effective interest rate	5.08%
Notes payable, maturity date	2015
5.75% Notes Due 2017 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	246,400,000	246,300,000
Notes payable, effective interest rate	5.75%
Notes payable, maturity date	2017
4.70% Notes Due 2021 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	248,800,000	248,700,000
Notes payable, effective interest rate	4.70%
Notes payable, maturity date	2021
3.07% Notes Due 2022 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	346,500,000	346,300,000
Notes payable, effective interest rate	3.07%
Notes payable, maturity date	2022
5.00% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	247,600,000	247,500,000
Notes payable, effective interest rate	5.00%
Notes payable, maturity date	2023
1.00% - 6.00% Conventional Mortgage Notes, Due 2014 - 2045 | Secured Debt [Member]
Total secured notes payable	906,400,000	934,600,000
Tax-Exempt Mortgage Note Due 2028 (1.26% floating rate) | Secured Debt [Member]
Total secured notes payable	36,600,000	37,700,000
Notes payable, effective interest rate	1.26%
Notes payable, maturity date	2028
Other Floating Rate Debt Included In Secured Notes (1.00%) | Secured Debt [Member]
Total secured notes payable	175,000,000	175,000,000
Minimum [Member] | 1.00% - 6.00% Conventional Mortgage Notes, Due 2014 - 2045 | Secured Debt [Member]
Notes payable, effective interest rate	1.00%
Notes payable, maturity date	2014
Minimum [Member] | Other Floating Rate Debt Included In Secured Notes (1.00%) | Secured Debt [Member]
Notes payable, effective interest rate	1.00%
Maximum [Member] | 1.00% - 6.00% Conventional Mortgage Notes, Due 2014 - 2045 | Secured Debt [Member]
Notes payable, effective interest rate	6.00%
Notes payable, maturity date	2045
Line of Credit [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 183,000,000	$ 0