Derivative and Other Fair Value Instruments (Details Textuals) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Mortgage notes payable	$ 5,090,960,000	$ 5,174,166,000
Notes, net	5,420,646,000	5,477,088,000
Debt Instrument, Fair Value Disclosure	5,100,000,000	5,100,000,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(165,000,000)	(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
Debt Instrument, Face Amount	750,000,000
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	100,000
Interest Expense	1,300,000
ChargesHedgeTerminationActivities	800,000
Total Note Payable and Long-Term Line of Credit [Member]
Derivative [Line Items]
Notes, net	5,900,000,000	5,600,000,000
Debt Instrument, Fair Value Disclosure	6,200,000,000	5,900,000,000
Fair Value Hedging maturity date 2019 [Member]
Derivative [Line Items]
Derivative, Variable Interest Rate	0.61%
London Interbank Offered Rate (LIBOR) [Member] | Fair Value Hedging maturity date 2019 [Member]
Derivative [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 0.61%
2.375% Notes [Member]
Derivative [Line Items]
Proceeds from Issuance of Unsecured Debt	$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.375%
Maturity Date	Jul. 01, 2019