Disclosure about Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Financial Assets:
Loans receivable, Carrying Amount	$ 312,356		$ 895,665
Available-for-sale equity investments, Carrying Amount	1,297		1,384
Available-for-sale equity investments, Fair Value	1,297	[1]	1,384
Cash and cash equivalents, Carrying Amount	512,472		1,033,764	204,895	163,482
Cash and cash equivalents, Fair Value	512,472		1,033,764
Foreign currency forward contract, asset	83,068	[2]
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	0		0	393,000
Borrowings under unsecured lines of credit arrangements, Fair Value	0		0
Senior unsecured notes, Carrying Amount	6,604,979		6,114,151
Senior unsecured notes, Fair Value	7,150,317		6,793,424
Secured debt, Carrying Amount	2,875,606		2,336,196
Secured debt, Fair Value	3,020,191		2,515,145
Interest rate swap agreements, Carrying Amount	135		264
Interest rate swap agreements, Fair Value	135		264
Foreign currency forward contact, liability			7,247
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	121,215		87,955
Loans receivable, Fair value	122,244		88,975
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	191,141		807,710
Loans receivable, Fair value	$ 198,391		$ 820,195

[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.