Notes Payable (Summary Of Indebtedness) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Unsecured notes payable	$ 1,380,952,000	$ 1,380,755,000
Total secured notes payable	1,050,154,000	1,051,357,000
Total notes payable	2,431,100,000	2,432,100,000
Senior Unsecured Notes [Member]
Unsecured notes payable	1,380,900,000	1,380,700,000
5.93% Notes Due 2012 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	189,600,000	189,600,000
Notes payable, effective interest rate	5.93%
Notes payable, maturity date	2012
5.45% Notes Due 2013 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	199,800,000	199,700,000
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	249,400,000	249,300,000
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	246,200,000	246,200,000
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	248,600,000	248,600,000
Notes payable, effective interest rate	4.70%
Notes payable, maturity date	2021
5.00% Notes Due 2023 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	247,300,000	247,300,000
Notes payable, effective interest rate	5.00%
Notes payable, maturity date	2023
0.89% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member] | Secured Debt [Member]
Total secured notes payable	1,011,500,000	1,012,300,000
1.32% Tax-Exempt Mortgage Note Due 2028 [Member] | Secured Debt [Member]
Total secured notes payable	38,700,000	39,100,000
Notes payable, effective interest rate	1.32%
Notes payable, maturity date	2028
Floating Rate Tax-Exempt Debt Included In Secured Notes (1.32%) [Member] | Secured Debt [Member]
Total secured notes payable	38,700,000	39,100,000
Notes payable, effective interest rate	1.32%
Floating Rate Debt Included In Secured Notes (0.89% - 1.68%) [Member] | Secured Debt [Member]
Total secured notes payable	206,500,000	206,400,000
Minimum [Member] | 0.89% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate	0.89%
Notes payable, maturity date	2012
Minimum [Member] | Floating Rate Debt Included In Secured Notes (0.89% - 1.68%) [Member] | Secured Debt [Member]
Notes payable, effective interest rate	0.89%
Maximum [Member] | 0.89% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate	6.00%
Notes payable, maturity date	2045
Maximum [Member] | Floating Rate Debt Included In Secured Notes (0.89% - 1.68%) [Member] | Secured Debt [Member]
Notes payable, effective interest rate	1.69%
Line of Credit [Member] | Commercial Banks [Member]
Unsecured notes payable	$ 0	$ 0