Mortgages Notes and Margins Payable (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Loans	Jun. 30, 2011	Dec. 31, 2011
Mortgages, Notes and Margins Payable (Textual) [Abstract]
Mortgage loans outstanding			$ 6,078,271		$ 5,812,595
Weighted average interest rate for mortgage loans	5.20%		5.20%		5.20%
Mortgage discount amount	33,219		33,219		30,741
Refinancing debt, outstanding			583,260
Number of mortgage loans does not require compliance with covenants			8
Carrying amount of mortgage loan	122,513		122,513
Net loans collateralized by the underlying properties at carrying value	96,762		96,762
Amount stated maturities of mortgage loans year, 2011			12,100
Amount stated maturities of mortgage loans year, 2012			24,970
Amount stated maturities of mortgage loans year, 2016			9,757
Amount stated maturities of mortgage loans year, 2017			75,686
Payable for securities purchased on margin	149,979		149,979		120,858
Margin rate on securities	0.59%		0.59%		0.621%
Interest expense	$ 228	$ 427	$ 84	$ 169