Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 3,362,573	$ 3,600,025
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	1,451,601
Unsecured notes | Notes payable maturing in 2012
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	201,601
Stated interest rate of unsecured notes (as a percent)	6.125%
Unsecured notes | Notes payable maturing in 2013
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	100,000
Stated interest rate of unsecured notes (as a percent)	4.95%
Unsecured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	150,000
Stated interest rate of unsecured notes (as a percent)	5.375%
Unsecured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	5.75%
Unsecured notes | Notes payable maturing in 2017
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	5.70%
Unsecured notes | Notes payable maturing in 2020
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	6.10%
Unsecured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	3.95%
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	182,659
Mortgage notes payable and unsecured notes	1,728,313
Secured notes | Notes payable maturing in 2012
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,966
Secured notes | Notes payable maturing in 2013
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	13,376
Mortgage notes payable and unsecured notes	223,473
Secured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	14,284
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	12,170
Mortgage notes payable and unsecured notes	406,019
Secured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	12,807
Secured notes | Notes payable maturing in 2017
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	13,709
Mortgage notes payable and unsecured notes	18,300
Secured notes | Notes payable maturing in 2018
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	14,330
Mortgage notes payable and unsecured notes	11,073
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	2,597
Mortgage notes payable and unsecured notes	610,813
Secured notes | Notes payable maturing in 2020
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	2,768
Secured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	2,952
Secured notes | Notes payable with maturities after 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	86,700
Mortgage notes payable and unsecured notes	$ 458,635