Disclosure about Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Financial Assets:
Loans receivable, Carrying Amount	$ 300,000		$ 292,507
Available-for-sale equity investments, Carrying Amount	942		980
Available-for-sale equity investments, Fair Value	942	[1]	980
Cash and cash equivalents, Carrying Amount	204,895		163,482	328,758	131,570
Cash and cash equivalents, Fair Value	204,895		163,482
Foreign currency forward contract	6,916	[2]
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	393,000		610,000	0
Borrowings under unsecured lines of credit arrangements, Fair Value	393,000		610,000
Senior unsecured notes, Carrying Amount	4,910,871		4,434,107
Senior unsecured notes, Fair Value	5,425,706		4,709,736
Secured debt, Carrying Amount	2,299,674		2,112,649
Secured debt, Fair Value	2,461,438		2,297,278
Interest rate swap agreements, Carrying Amount	979		2,854
Interest rate swap agreements, Fair Value	979		2,854
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	63,249		63,934
Loans receivable, Fair value	65,993		64,194
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	236,751		228,573
Loans receivable, Fair value	$ 246,785		$ 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.