Financial Instruments (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Debt instruments with carrying value different than estimated fair values
Senior notes, Carrying Amount	$ 1,938,255	$ 2,139,718
Revolving Credit Facilities and Term Loans, Carrying Amount	825,968	642,421
Mortgage payable and other indebtedness, Carrying Amount	1,372,694	1,322,445
Long-term debt, carrying amount total	4,136,917	4,104,584
Senior notes, Fair Value	2,181,764	2,282,818
Revolving Credit Facilities and Term Loans, Fair Value	824,073	641,854
Mortgage payable and other indebtedness, Fair Value	1,406,107	1,352,142
Long-term debt, fair value total	$ 4,411,944	$ 4,276,814