Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Unsecured notes	Nov. 30, 2012 Unsecured notes	Dec. 31, 2012 Unsecured notes Minimum	Dec. 31, 2012 Unsecured notes Maximum	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2013	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2014	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2016	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2017	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2020	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2021	Dec. 31, 2012 Unsecured notes Notes payable maturing in 2022	Dec. 31, 2012 Unsecured notes Notes payable with maturities after 2022	Dec. 31, 2012 Secured notes	Dec. 31, 2012 Secured notes Notes payable maturing in 2013	Dec. 31, 2012 Secured notes Notes payable maturing in 2014	Dec. 31, 2012 Secured notes Notes payable maturing in 2015	Dec. 31, 2012 Secured notes Notes payable maturing in 2016	Dec. 31, 2012 Secured notes Notes payable maturing in 2017	Dec. 31, 2012 Secured notes Notes payable maturing in 2018	Dec. 31, 2012 Secured notes Notes payable maturing in 2019	Dec. 31, 2012 Secured notes Notes payable maturing in 2020	Dec. 31, 2012 Secured notes Notes payable maturing in 2021	Dec. 31, 2012 Secured notes Notes payable maturing in 2022	Dec. 31, 2012 Secured notes Notes payable with maturities after 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments															$ 175,691	$ 13,375	$ 14,284	$ 12,170	$ 12,807	$ 13,709	$ 14,330	$ 2,597	$ 2,768	$ 2,952	$ 3,147	$ 83,552
Mortgage notes payable and unsecured notes	$ 3,854,068	$ 3,600,025	$ 1,950,000				$ 100,000	$ 150,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 450,000	$ 250,000	$ 1,728,377	$ 223,473		$ 406,083		$ 18,300	$ 11,073	$ 610,813				$ 458,635
Stated interest rate of unsecured notes (as a percent)				6.125%			4.95%	5.375%	5.75%	5.70%	6.10%	3.95%	2.95%	2.85%
Percentage of principal amount at which the entity may redeem some or all of the notes	100.00%
Debt instrument, basis spread on variable rate (as a percent)					0.25%	0.45%