Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 5,972,282	$ 3,854,068
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	2,250,000
Stated interest rate of unsecured notes (as a percent)	3.625%
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 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 6.100 Percent Maturities 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 3.625 Percent Maturities 2020
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	400,000
Stated interest rate of unsecured notes (as a percent)	3.625%
Unsecured notes | Notes Payable 2.950 Percent Maturities 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 2.850 Percent Maturities 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	210,215
Mortgage notes payable and unsecured notes	3,512,067
Secured notes | Notes payable maturing in 2013
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,395
Secured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	18,056
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	16,722
Mortgage notes payable and unsecured notes	904,011
Secured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,951
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	19,033
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	18,398
Mortgage notes payable and unsecured notes	77,189
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,125
Mortgage notes payable and unsecured notes	610,814
Secured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,965
Mortgage notes payable and unsecured notes	27,844
Secured notes | Notes Payable 6.100 Percent Maturities 2020
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,190
Mortgage notes payable and unsecured notes	50,824
Secured notes | Notes Payable 2.950 Percent Maturities 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,332
Secured notes | Notes Payable 2.850 Percent Maturities 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	90,048
Mortgage notes payable and unsecured notes	$ 1,114,639