Disclosure about Fair Value of Financial Instruments (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010		Jun. 30, 2010	Dec. 31, 2009
Financial Assets:
Loans receivable, Carrying Amount	$ 326,651,000	$ 436,580,000	[1]
Available-for-sale equity investments, Carrying Amount	1,189,000	1,103,000
Available-for-sale equity investments, Fair Value	1,189,000	1,103,000
Cash and cash equivalents, Carrying Amount	328,758,000	131,570,000	[1]	55,423,000	35,476,000
Cash and cash equivalents, Fair Value	328,758,000	131,570,000
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	0	300,000,000	[1]	206,000,000
Borrowings under unsecured lines of credit arrangements, Fair Value	0	300,000,000
Senior unsecured notes, Carrying Amount	4,429,992,000	3,034,949,000	[1]
Senior unsecured notes, Fair Value	4,464,930,000	3,267,638,000
Secured debt, Carrying Amount	1,889,873,000	1,125,906,000	[1]
Secured debt, Fair Value	1,977,949,000	1,178,081,000
Interest rate swap agreements, Carrying Amount	1,414,000	482,000
Interest rate swap agreements, Fair Value	1,414,000	482,000
Mortgage Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	75,171,000	109,283,000
Loans receivable, Fair value	81,005,000	111,255,000
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	251,480,000	327,297,000
Loans receivable, Fair value	$ 253,360,000	$ 333,003,000

[1] NOTE: The consolidated balance sheet at December 31, 2010 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.