Derivative and Other Fair Value Instruments (Details Textuals) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Mortgage notes payable	$ 6,247,612,000	$ 3,898,369,000
Notes, net	5,475,954,000	4,630,875,000
Fair value of mortgage notes payable	6,400,000,000	4,300,000,000
Fair value of unsecured notes payable including line of credit	5,800,000,000	5,200,000,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	(170,252,519)	(194,700,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	21,600,000
Payments for (Proceeds from) Hedge, Investing Activities	44,700,000
Proceeds from Issuance of Unsecured Debt	500,000,000
Interest Expense	700,000
Deferred accumulated other comprehensive (loss)	$ 44,000,000