Disclosure about Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Financial Assets:
Loans receivable, Carrying Amount	$ 284,908		$ 292,507
Available-for-sale equity investments, Carrying Amount	1,172		980
Available-for-sale equity investments, Fair Value	1,172	[1]	980
Cash and cash equivalents, Carrying Amount	1,382,252		163,482	136,676	131,570
Cash and cash equivalents, Fair Value	1,382,252		163,482
Foreign currency forward contract	(4,595)	[2]
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	0		610,000	390,000
Borrowings under unsecured lines of credit arrangements, Fair Value	0		610,000
Senior unsecured notes, Carrying Amount	4,921,712		4,434,107
Senior unsecured notes, Fair Value	5,568,956		4,709,736
Secured debt, Carrying Amount	2,314,717		2,112,649
Secured debt, Fair Value	2,486,885		2,297,278
Interest rate swap agreements, Carrying Amount	740		2,854
Interest rate swap agreements, Fair Value	740		2,854
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	61,591		63,934
Loans receivable, Fair value	63,647		64,194
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	223,317		228,573
Loans receivable, Fair value	$ 231,388		$ 231,308

[1] (1) Unrealized gains or losses on equity investments are recorded in accumulated other comprehensive income (loss) at each measurement date.

[2] (2) Please see Note 11 for additional information.