Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 5,576,030	$ 3,854,068
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	1,850,000
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%
Unsecured notes | Notes payable maturing in 2022
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	450,000
Stated interest rate of unsecured notes (as a percent)	2.95%
Unsecured notes | Notes payable with maturities after 2022
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	2.85%
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	230,978
Mortgage notes payable and unsecured notes	3,495,052
Secured notes | Notes payable maturing in 2013
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	9,723
Secured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,608
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	18,358
Mortgage notes payable and unsecured notes	904,014
Secured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,708
Mortgage notes payable and unsecured notes	16,255
Secured notes | Notes payable maturing in 2017
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	20,865
Mortgage notes payable and unsecured notes	710,491
Secured notes | Notes payable maturing in 2018
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	20,200
Mortgage notes payable and unsecured notes	76,759
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	8,761
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	7,934
Mortgage notes payable and unsecured notes	50,825
Secured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,779
Mortgage notes payable and unsecured notes	27,844
Secured notes | Notes payable maturing in 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	8,242
Secured notes | Notes payable with maturities after 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	89,800
Mortgage notes payable and unsecured notes	$ 1,098,051