Derivative and Other Fair Value Instruments (Details Textuals) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Mortgage notes payable	$ 4,119,181,000	$ 4,685,134,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(113,900,000)	(151,800,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	21,900,000
Payments for (Proceeds from) Hedge, Investing Activities			$ 2,000,000
Proceeds from Issuance of Unsecured Debt			750,000,000
Interest Expense			1,300,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net		3,000,000	(100,000)
Charges Hedge Termination Activities			$ 800,000
2.85% Notes [Member]
Derivative [Line Items]
Payments for (Proceeds from) Hedge, Investing Activities	4,700,000
Proceeds from Issuance of Unsecured Debt	500,000,000
Interest Expense	9,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	(74,000)
Charges Hedge Termination Activities	4,600,000
3.375% Notes [Member] [Member]
Derivative [Line Items]
Payments for (Proceeds from) Hedge, Investing Activities		15,100,000
Proceeds from Issuance of Unsecured Debt	450,000,000	450,000,000
Interest Expense		1,200,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net		(30,000)
Charges Hedge Termination Activities		13,900,000
Secured Debt [Member]
Derivative [Line Items]
Mortgage notes payable	4,100,000,000	4,700,000,000
Debt Instrument, Fair Value Disclosure	4,200,000,000	4,600,000,000
Unsecured Debt [Member]
Derivative [Line Items]
Notes, net	4,900,000,000	6,200,000,000
Debt Instrument, Fair Value Disclosure	$ 5,000,000,000	$ 6,500,000,000