Disclosure about Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financial Assets:
Loans receivable, Carrying Amount	$ 332,146,000		$ 895,665,000
Available-for-sale equity investments, Carrying Amount	1,211,000		1,384,000
Available-for-sale equity investments, Fair Value	1,211,000		1,384,000
Cash and cash equivalents, Carrying Amount	158,780,000		1,033,764,000	163,482,000	131,570,000
Cash and cash equivalents	158,780,000		1,033,764,000
Interest rate derivative, Fair Value, Asset	38,000	[1]	0
Financial Liabilities:
Borrowings under unsecured lines of credit arrangements, Carrying Amount	130,000,000		0	610,000,000
Borrowings under unsecured lines of credit arrangements, Fair Value	130,000,000		0
Senior unsecured notes, Carrying Amount	7,379,308,000		6,114,151,000
Senior unsecured notes, Fair Value	7,743,730,000		6,793,424,000
Secured debt, Carrying Amount	3,058,248,000		2,336,196,000
Secured debt, Fair Value	3,168,775,000		2,515,145,000
Interest rate swap agreements, Fair Value	0		264,000
Foreign currency forward contract	(11,637,000)	[1]	7,247,000
Mortgage Loans [Member]
Financial Assets:
Loans receivable, Carrying Amount	146,987,000		87,955,000
Loans receivable, Fair value	148,088,000		88,975,000
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	185,159,000		807,710,000
Loans receivable, Fair value	$ 188,920,000		$ 820,195,000

[1] Please see Note 11 for additional information.