Notes Payable, Unsecured Notes and Credit Facility (Details 3) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2015	Apr. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	May. 31, 2015	Dec. 31, 2014
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt			$ 18,987	$ 0	$ 26,736	$ (412)
Mortgage notes payable and unsecured notes			6,288,982		6,288,982			$ 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			80,923		80,923
Mortgage notes payable and unsecured notes			2,633,059		2,633,059
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			4,295		4,295
Mortgage notes payable and unsecured notes			103,743		103,743
Secured notes | Notes Payable Maturities 2016 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			16,164		16,164
Mortgage notes payable and unsecured notes			16,256		16,256
Secured notes | Notes Payable Maturities 2017 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			17,166		17,166
Mortgage notes payable and unsecured notes			709,991		709,991
Secured notes | Notes Payable Maturities 2018 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			16,364		16,364
Mortgage notes payable and unsecured notes			76,673		76,673
Secured notes | Notes payable maturing in 2019
Notes Payable, Unsecured Notes and Credit Facility
Gains (Losses) on Extinguishment of Debt	$ (259)
Secured notes payments			5,099		5,099
Mortgage notes payable and unsecured notes			588,428		588,428
Secured notes | Notes Payable Maturities 2020 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			4,057		4,057
Mortgage notes payable and unsecured notes			50,825		50,825
Secured notes | Notes Payable Maturities 2021 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			4,017		4,017
Mortgage notes payable and unsecured notes			27,844		27,844
Secured notes | Notes payable maturing in 2022
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			4,295		4,295
Secured notes | Notes payable maturing in 2023
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			4,578		4,578
Secured notes | Notes Payable Maturities 2024 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Secured notes payments			4,888		4,888
Secured notes | Notes payable with maturities after 2023
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes			1,059,299		1,059,299
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
Unsecured notes | Notes Payable Maturities 2016 [Member]
Notes Payable, Unsecured Notes and Credit Facility
Mortgage notes payable and unsecured notes			$ 250,000		$ 250,000
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			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
Stated interest rate of unsecured notes (as a percent)			3.45%		3.45%