Disclosures About Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Summary of financial instruments
Senior unsecured notes	$ 6,826,884	$ 6,963,375
Mortgage debt	1,229,773	1,396,485
Other debt	73,020	74,909
Carrying Value
Summary of financial instruments
Loans receivable, net	375,717	366,001
Marketable debt securities	251,888	244,089
Marketable equity securities	27
Term loan	234,352	226,858
Senior unsecured notes	6,826,884	6,963,375
Mortgage debt	1,229,773	1,396,485
Other debt	73,020	74,909
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	182	114
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets	2,576	2,325
Derivative liabilities	(8,327)	8,384
Carrying Value | Currency swap contracts
Summary of financial instruments
Derivative liabilities	(3,756)	2,756
Fair Value | Level 1
Summary of financial instruments
Marketable debt securities	293,188	280,850
Marketable equity securities	27
Senior unsecured notes	7,441,232	7,405,817
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	387,621	373,441
Term loan	234,352	226,858
Mortgage debt	1,273,673	1,421,214
Other debt	73,020	74,909
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	182	114
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets	2,576	2,325
Derivative liabilities	(8,327)	8,384
Fair Value | Currency swap contracts | Level 2
Summary of financial instruments
Derivative liabilities	$ (3,756)	$ 2,756