Notes Payable (Summary Of Indebtedness) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Unsecured notes payable	$ 1,649,041,000	$ 1,588,798,000
Total secured notes payable	940,881,000	941,968,000
Total notes payable	2,589,900,000	2,530,800,000
Senior Unsecured Notes [Member]
Unsecured notes payable	1,589,000,000	1,588,800,000
Tax Exempt Mortgage Note Due 2028 (1.31% floating rate) | Secured Debt [Member]
Total secured notes payable	35,900,000	36,300,000
Notes payable, effective interest rate	1.31%
Notes payable, maturity date	2028
5.08% Notes Due 2015 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	249,800,000	249,700,000
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	246,400,000
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	248,900,000	248,800,000
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	346,700,000	346,700,000
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	247,700,000	247,700,000
Notes payable, effective interest rate	5.00%
Notes payable, maturity date	2023
4.27% Notes Due 2024 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	249,500,000	249,500,000
Notes payable, effective interest rate	4.27%
Notes payable, maturity date	2024
Secured Notes 0.93% to 6.00% Conventional Mortgage Notes Due 2014 to 2045 [Member] | Secured Debt [Member]
Total secured notes payable	905,000,000	905,700,000
Secured Notes 0.93% Floating Rate [Member] | Secured Debt [Member]
Total secured notes payable	175,000,000	175,000,000
Minimum [Member] | Secured Notes 0.93% to 6.00% Conventional Mortgage Notes Due 2014 to 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate	0.93%
Notes payable, maturity date	2014
Minimum [Member] | Secured Notes 0.93% Floating Rate [Member] | Secured Debt [Member]
Notes payable, effective interest rate	0.93%
Maximum [Member] | Secured Notes 0.93% to 6.00% Conventional Mortgage Notes Due 2014 to 2045 [Member] | 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	$ 60,000,000	$ 0