Disclosures About Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of financial instruments
Marketable equity securities	$ 24,800	$ 17,100
Bank line of credit		454,000
Senior unsecured notes	6,712,624	5,416,063
Mortgage debt	1,676,544	1,715,039
Other debt	81,958	87,985
Carrying Amount
Summary of financial instruments
Loans receivable, net	276,030	110,253
Marketable debt securities	222,809
Marketable equity securities	24,829	17,053
Bank line of credit		454,000
Term loan	222,694
Senior unsecured notes	6,712,624	5,416,063
Mortgage debt	1,676,544	1,764,571
Other debt	81,958	87,985
Carrying Amount | Warrants
Summary of financial instruments
Derivative assets	670	1,334
Carrying Amount | Interest-rate swap contracts
Summary of financial instruments
Derivative assets	89
Derivative liabilities	12,699	12,123
Carrying Amount | Currency swap liabilities
Summary of financial instruments
Derivative liabilities	2,641
Fair Value
Summary of financial instruments
Loans receivable, net	279,850	111,073
Marketable debt securities	234,137
Marketable equity securities	24,829	17,053
Bank line of credit		454,000
Term loan	222,694
Senior unsecured notes	7,432,012	5,819,304
Mortgage debt	1,771,155	1,870,070
Other debt	81,958	87,985
Fair Value | Warrants
Summary of financial instruments
Derivative assets	670	1,334
Fair Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets	89
Derivative liabilities	12,699	12,123
Fair Value | Currency swap liabilities
Summary of financial instruments
Derivative liabilities	$ 2,641