Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 6,481,291	$ 6,448,138
Percentage of principle amount at which the entity may redeem some or all of the notes	100.00%
Unsecured notes
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 3,875,000
Unsecured notes | Minimum
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)	0.25%
Unsecured notes | Maximum
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 300,000
Unsecured notes | Term loan | LIBOR
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage 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
Mortgage 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%
Unsecured notes | Notes Payable 3.450 Maturities 2025 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 525,000
Stated interest rate of unsecured notes (as a percent)	3.45%
Unsecured notes | Notes Payable 3.500 Maturities 2025 [Member]
Mortgage 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
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	$ 296,813
Mortgage notes payable and unsecured notes	2,309,478
Secured notes | Notes payable maturing in 2016
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	16,164
Mortgage notes payable and unsecured notes	16,255
Secured notes | Notes payable maturing in 2017
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,166
Mortgage notes payable and unsecured notes	709,891
Secured notes | Notes payable maturing in 2018
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	16,236
Mortgage notes payable and unsecured notes	76,950
Secured notes | Notes payable maturing in 2019
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,696
Mortgage notes payable and unsecured notes	588,429
Secured notes | Notes Payable 6.100 Percent Maturities 2020
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	3,624
Mortgage notes payable and unsecured notes	50,825
Secured notes | Notes payable maturing in 2021
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	3,551
Mortgage notes payable and unsecured notes	27,844
Secured notes | Notes payable maturing in 2022
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	3,795
Mortgage notes payable and unsecured notes	0
Secured notes | Notes Payable 4.200 Maturities 2023
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,040
Secured notes | Notes Payable 2024
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,310
Mortgage notes payable and unsecured notes	0
Secured notes | Notes Payable Maturities 2025 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	4,553
Mortgage notes payable and unsecured notes	84,835
Secured notes | Notes payable with maturities after 2024
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	218,678
Mortgage notes payable and unsecured notes	$ 754,449