Disclosures About Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of financial instruments
Marketable equity securities		$ 24,829
Bank line of credit	285,000
Senior unsecured notes	6,565,934	6,712,624
Mortgage debt	1,410,407	1,676,544
Other debt	77,503	81,958
Carrying Value
Summary of financial instruments
Loans receivable, net	390,803	276,030
Marketable debt securities	238,834	222,809
Marketable equity securities		24,829
Bank line of credit	285,000
Term loan	221,748	222,694
Senior unsecured notes	6,565,934	6,712,624
Mortgage debt	1,410,407	1,676,544
Other debt	77,503	81,958
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	180	670
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets	1,564	89
Derivative liabilities	9,283	12,699
Carrying Value | Currency swap contracts
Summary of financial instruments
Derivative liabilities	1,897	2,641
Fair Value | Level 1
Summary of financial instruments
Marketable debt securities	272,745	234,137
Marketable equity securities		24,829
Senior unsecured notes	7,027,982	7,432,012
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	402,243	279,850
Bank line of credit	285,000
Term loan	221,748	222,694
Mortgage debt	1,452,732	1,771,155
Other debt	77,503	81,958
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	180	670
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets	1,564	89
Derivative liabilities	9,283	12,699
Fair Value | Currency swap contracts | Level 2
Summary of financial instruments
Derivative liabilities	$ 1,897	$ 2,641