Disclosures About Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of financial instruments
Bank line of credit	$ 14,000
Senior unsecured notes	6,563,749	6,712,624
Mortgage debt	1,680,792	1,676,544
Other debt	78,836	81,958
Carrying Amount
Summary of financial instruments
Loans receivable, net	291,870	276,030
Marketable debt securities	208,214	222,809
Marketable equity securities		24,829
Bank line of credit	14,000
Term loan	208,213	222,694
Senior unsecured notes	6,563,749	6,712,624
Mortgage debt	1,680,792	1,676,544
Other debt	78,836	81,958
Carrying Amount | Warrants
Summary of financial instruments
Derivative assets	775	670
Carrying Amount | Interest-rate swap contracts
Summary of financial instruments
Derivative assets		89
Derivative liabilities	12,421	12,699
Carrying Amount | Currency swap contracts
Summary of financial instruments
Derivative assets	2,490
Derivative liabilities		2,641
Fair Value
Summary of financial instruments
Loans receivable, net	294,724	279,850
Marketable debt securities	226,337	234,137
Marketable equity securities		24,829
Bank line of credit	14,000
Term loan	208,213	222,694
Senior unsecured notes	7,303,804	7,432,012
Mortgage debt	1,763,165	1,771,155
Other debt	78,836	81,958
Fair Value | Warrants
Summary of financial instruments
Derivative assets	775	670
Fair Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets		89
Derivative liabilities	12,421	12,699
Fair Value | Currency swap contracts
Summary of financial instruments
Derivative assets	2,490
Derivative liabilities		$ 2,641