Financial Instruments (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Debt instruments with carrying value different than estimated fair values
Senior notes	$ 1,977,512	$ 2,139,718
Revolving credit facilities and term loans, Carrying amount	755,895	642,421
Mortgage and other secured indebtedness	1,362,378	1,322,445
Total indebtedness	4,095,785	4,104,584
Senior notes, Fair Value	2,210,960	2,282,818
Revolving Credit Facilities and term loans, Fair Value	755,397	641,854
Mortgage payable and other indebtedness, Fair Value	1,389,444	1,352,142
Long-term debt, Fair Value Total	$ 4,355,801	$ 4,276,814