Notes Payable (Summary Of Indebtedness) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Unsecured notes payable	$ 1,538,212,000	$ 1,380,755,000
Total secured notes payable	972,256,000	1,051,357,000
Total notes payable	2,510,500,000
Notes payable, effective interest rate	3.07%
Value Of Real Estate Assets At Cost Subject To Secured Notes	1,584,700,000	1,651,000,000
5.93% Notes Due 2012 [Member] | Senior Unsecured Notes [Member]
Unsecured notes payable	0	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	199,900,000	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	249,500,000	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	246,300,000	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	248,700,000	248,600,000
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	346,300,000	0
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	247,500,000	247,300,000
Notes payable, effective interest rate	5.00%
Notes payable, maturity date	2023
1.02% - 6.00% Conventional Mortgage Notes, Due 2013 - 2045 [Member] | Secured Debt [Member]
Total secured notes payable	934,600,000	1,012,300,000
1.37% Tax-Exempt Mortgage Note Due 2028 [Member] | Secured Debt [Member]
Total secured notes payable	37,700,000	39,100,000
Notes payable, effective interest rate	1.37%
Notes payable, maturity date	2028
Floating Rate Tax-Exempt Debt Included In Secured Notes (1.35%) [Member] | Secured Debt [Member]
Total secured notes payable	37,700,000	39,100,000
Notes payable, effective interest rate	1.37%
Floating Rate Debt Included In Secured Notes (0.96% - 1.69%) [Member] | Secured Debt [Member]
Total secured notes payable	$ 175,000,000	$ 206,400,000
Minimum [Member] | 1.02% - 6.00% Conventional Mortgage Notes, Due 2013 - 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate	1.02%
Notes payable, maturity date	2013
Minimum [Member] | Floating Rate Debt Included In Secured Notes (0.96% - 1.69%) [Member] | Secured Debt [Member]
Notes payable, effective interest rate	1.02%
Maximum [Member] | 1.02% - 6.00% Conventional Mortgage Notes, Due 2013 - 2045 [Member] | Secured Debt [Member]
Notes payable, effective interest rate	6.00%
Notes payable, maturity date	2045