Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 5,784,508	$ 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	233,623
Mortgage notes payable and unsecured notes	3,700,885
Secured notes | Notes payable maturing in 2013
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	13,774
Mortgage notes payable and unsecured notes	270,144
Secured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,527
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,716
Mortgage notes payable and unsecured notes	904,005
Secured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	18,482
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	19,604
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	19,259
Mortgage notes payable and unsecured notes	63,261
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,802
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	8,256
Secured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	8,120
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	8,602
Secured notes | Notes payable with maturities after 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	92,481
Mortgage notes payable and unsecured notes	$ 1,098,072