Derivative and Other Fair Value Instruments (Details Textuals) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Mortgage notes payable	$ 5,086,515,000	$ 5,174,166,000
Notes, net	5,425,346,000	5,477,088,000
Fair value of mortgage notes payable	5,100,000,000	5,100,000,000
Fair value of unsecured notes payable including line of credit	6,100,000,000	5,900,000,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(172,200,000)	(155,800,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	22,000,000
Payments for (Proceeds from) Hedge, Investing Activities	2,000,000	44,700,000
Debt Instrument, Face Amount	750,000,000
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	100,000
Proceeds from Issuance of Unsecured Debt		500,000,000
Interest Expense	1,300,000	700,000
Charges Hedge Termination Activities	800,000	44,000,000
Total Note Payable and Long-Term Line of Credit [Member]
Derivative [Line Items]
Notes, net	$ 5,800,000,000	$ 5,600,000,000