Notes Payable, Unsecured Notes and Credit Facility (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 6,448,138	$ 6,019,571
Percentage of principle 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	3,000,000
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 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 | Term loan
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	250,000
Unsecured notes | Term loan | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)	1.45%
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 4.200 Maturities 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)	4.20%
Unsecured notes | Notes Payable 2.850 Maturities 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)	2.85%
Unsecured notes | Notes Payable 2024
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	300,000
Stated interest rate of unsecured notes (as a percent)	3.50%
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	114,099
Mortgage notes payable and unsecured notes	3,334,039
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,873
Mortgage notes payable and unsecured notes	586,705
Secured notes | Notes payable maturing in 2016
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,037
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	20,255
Mortgage notes payable and unsecured notes	710,291
Secured notes | Notes payable maturing in 2018
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,649
Mortgage notes payable and unsecured notes	76,937
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,141
Mortgage notes payable and unsecured notes	658,447
Secured notes | Notes Payable 6.100 Percent Maturities 2020
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,209
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	5,984
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	6,351
Mortgage notes payable and unsecured notes	0
Secured notes | Notes Payable 4.200 Maturities 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,742
Secured notes | Notes Payable 2024
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,858
Mortgage notes payable and unsecured notes	0
Secured notes | Notes payable with maturities after 2024
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	0
Mortgage notes payable and unsecured notes	$ 1,206,736