Derivative and Other Fair Value Instruments (Details Textuals) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Jun. 30, 2016
Derivative [Line Items]
Mortgage notes payable	$ 4,952,579		$ 5,086,515
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(163,400)		(172,200)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net				$ 23,800
Payments for (Proceeds from) Hedge, Investing Activities	15,100
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income	(30)	$ 91
Interest Expense	1,200
ChargesHedgeTerminationActivities	13,900
Unsecured Debt [Member]
Derivative [Line Items]
Unsecured Debt	5,900,000		5,800,000
Debt Instrument, Fair Value Disclosure	6,100,000		6,100,000
Secured Debt [Member]
Derivative [Line Items]
Mortgage notes payable	4,952,579
Debt Instrument, Fair Value Disclosure	4,900,000		$ 5,100,000
3.375% Notes [Member] [Member]
Derivative [Line Items]
Proceeds from Issuance of Unsecured Debt	$ 450,000