Notes Payable, Unsecured Notes and Credit Facility (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	6,493,278	$ 6,448,138
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	112,972
Mortgage notes payable and unsecured notes	3,330,306
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	13,580
Mortgage notes payable and unsecured notes	586,228
Secured notes | Notes Payable Maturities 2016 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,184
Mortgage notes payable and unsecured notes	16,256
Secured notes | Notes Payable Maturities 2017 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	20,387
Mortgage notes payable and unsecured notes	710,191
Secured notes | Notes Payable Maturities 2018 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	19,788
Mortgage notes payable and unsecured notes	76,940
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,287
Mortgage notes payable and unsecured notes	658,449
Secured notes | Notes Payable Maturities 2020 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,384
Mortgage notes payable and unsecured notes	50,825
Secured notes | Notes Payable Maturities 2021 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,491
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	6,927
Secured notes | Notes payable maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	7,377
Secured notes | Notes Payable Maturities 2024 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,567
Secured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	1,203,573
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	3,050,000
Unsecured notes | Notes Payable Maturities 2016 [Member]
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 Maturities 2017 [Member]
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	300,000
Unsecured notes | Term Loan | LIBOR
Notes Payable, Unsecured Notes and Credit Facility
Debt instrument variable rate	LIBOR
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 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)	4.20%
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 Maturities 2024 [Member]
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%