Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Jan. 31, 2012 5.5% notes payable maturing in 2012	Mar. 31, 2012 Unsecured notes	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2012	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2013	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2014	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2016	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2017	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2020	Mar. 31, 2012 Unsecured notes Notes payable maturing in 2021	Mar. 31, 2012 Secured notes	Mar. 31, 2012 Secured notes Notes payable maturing in 2012	Mar. 31, 2012 Secured notes Notes payable maturing in 2013	Mar. 31, 2012 Secured notes Notes payable maturing in 2014	Mar. 31, 2012 Secured notes Notes payable maturing in 2015	Mar. 31, 2012 Secured notes Notes payable maturing in 2016	Mar. 31, 2012 Secured notes Notes payable maturing in 2017	Mar. 31, 2012 Secured notes Notes payable maturing in 2018	Mar. 31, 2012 Secured notes Notes payable maturing in 2019	Mar. 31, 2012 Secured notes Notes payable maturing in 2020	Mar. 31, 2012 Secured notes Notes payable maturing in 2021	Mar. 31, 2012 Secured notes Notes payable with maturities after 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments												$ 185,965	$ 10,274	$ 13,376	$ 14,284	$ 12,170	$ 12,807	$ 13,709	$ 14,330	$ 2,597	$ 2,768	$ 2,952	$ 86,698
Mortgage notes payable and unsecured notes	$ 3,380,545	$ 3,600,025		$ 1,451,601	$ 201,601	$ 100,000	$ 150,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 1,742,979	$ 14,666	$ 223,473		$ 406,019		$ 18,300	$ 11,073	$ 610,813			$ 45,863
Stated interest rate of unsecured notes (as a percent)			5.50%		6.125%	4.95%	5.375%	5.75%	5.70%	6.10%	3.95%