Notes Payable, Unsecured Notes and Credit Facility (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 6,029,998	$ 3,854,068
Percentage of principal amount at which the entity may redeem some or all of the notes	100.00%
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	2,600,000
Unsecured notes | Minimum
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)	0.25%
Unsecured notes | Maximum
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)	0.45%
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 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 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 maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	350,000
Stated interest rate of unsecured notes (as a percent)	4.30%
Unsecured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Stated interest rate of unsecured notes (as a percent)	3.00%
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	207,235
Mortgage notes payable and unsecured notes	3,222,763
Secured notes | Notes payable maturing in 2014
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	16,986
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	603,044
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	77,432
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	610,811
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 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 maturing in 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,350
Secured notes | Notes payable maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,744
Secured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	85,771
Mortgage notes payable and unsecured notes	$ 1,126,062