Notes Payable, Unsecured Notes and Credit Facility (Details 3) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2015	Apr. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	May. 31, 2015	Dec. 31, 2014
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt			$ 7,749	$ (412)	$ 7,749	$ (412)
Mortgage notes payable and unsecured notes	$ 6,433,889		6,433,889		6,433,889			$ 6,448,138
Stated interest rate of unsecured notes (as a percent)							3.45%
Secured notes
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	96,872		96,872		96,872
Mortgage notes payable and unsecured notes	2,762,017		2,762,017		2,762,017
Secured notes | Notes payable maturing in 2015
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt		$ 8,724
Secured notes payments	8,443		8,443		8,443
Mortgage notes payable and unsecured notes	104,198		104,198		104,198
Secured notes | Notes Payable Maturities 2016 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,298		17,298		17,298
Mortgage notes payable and unsecured notes	16,256		16,256		16,256
Secured notes | Notes Payable Maturities 2017 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	18,365		18,365		18,365
Mortgage notes payable and unsecured notes	710,091		710,091		710,091
Secured notes | Notes Payable Maturities 2018 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	17,632		17,632		17,632
Mortgage notes payable and unsecured notes	76,940		76,940		76,940
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt	(259)
Secured notes payments	6,440		6,440		6,440
Mortgage notes payable and unsecured notes	588,428		588,428		588,428
Secured notes | Notes Payable Maturities 2020 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,475		5,475		5,475
Mortgage notes payable and unsecured notes	50,825		50,825		50,825
Secured notes | Notes Payable Maturities 2021 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,516		5,516		5,516
Mortgage notes payable and unsecured notes	27,844		27,844		27,844
Secured notes | Notes payable maturing in 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,881		5,881		5,881
Secured notes | Notes payable maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	6,255		6,255		6,255
Secured notes | Notes Payable Maturities 2024 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments	5,567		5,567		5,567
Secured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	1,187,435		1,187,435		1,187,435
Secured notes | Notes Payable Maturities 2027 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt	(263)
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	3,575,000		3,575,000		3,575,000
Unsecured notes | Notes Payable Maturities 2016 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 250,000		$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)	5.75%		5.75%		5.75%
Unsecured notes | Notes Payable Maturities 2017 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 250,000		$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)	5.70%		5.70%		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		$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)	6.10%		6.10%		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		$ 400,000		$ 400,000
Stated interest rate of unsecured notes (as a percent)	3.625%		3.625%		3.625%
Unsecured notes | Notes payable maturing in 2021
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 250,000		$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)	3.95%		3.95%		3.95%
Unsecured notes | Term Loan
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 300,000		$ 300,000		$ 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		$ 450,000		$ 450,000
Stated interest rate of unsecured notes (as a percent)	2.95%		2.95%		2.95%
Unsecured notes | Notes payable maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 350,000		$ 350,000		$ 350,000
Stated interest rate of unsecured notes (as a percent)	4.20%		4.20%		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		$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)	2.85%		2.85%		2.85%
Unsecured notes | Notes Payable Maturities 2024 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 300,000		$ 300,000		$ 300,000
Stated interest rate of unsecured notes (as a percent)	3.50%		3.50%		3.50%
Unsecured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes	$ 525,000		$ 525,000		$ 525,000
Stated interest rate of unsecured notes (as a percent)	3.45%		3.45%		3.45%