Disclosure about Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Financial Assets:
Loans receivable, Carrying Amount	$ 276,876		$ 895,665
Available-for-sale equity investments, Carrying Amount	1,555		1,384
Available-for-sale equity investments, Fair Value	1,555	[1]	1,384
Cash and cash equivalents, Carrying Amount	269,842		1,033,764	469,217	163,482
Cash and cash equivalents, Fair Value	269,842		1,033,764
Foreign currency forward contract	63,249	[2]
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	710,000		0	5,000
Borrowings under unsecured lines of credit arrangements, Fair Value	710,000		0
Senior unsecured notes, Carrying Amount	6,610,873		6,114,151
Senior unsecured notes, Fair Value	7,363,318		6,793,424
Secured debt, Carrying Amount	2,452,495		2,336,196
Secured debt, Fair Value	2,643,513		2,515,145
Interest rate swap agreements, Carrying Amount	200		264
Interest rate swap agreements, Fair Value	200		264
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	88,861		87,955
Loans receivable, Fair value	89,693		88,975
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	188,015		807,710
Loans receivable, Fair value	$ 199,985		$ 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.