Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value Measurements [Abstract]
Carrying Value of Mortgage and notes payable	$ 6,078,271	$ 5,812,595
Payable for securities purchased on margin	149,979	120,858
Estimated Fair Value of Mortgage and notes payable	5,937,002	5,524,022
Estimated Fair Value of Margins payable	$ 149,979	$ 120,858