Disclosure about Fair Value of Financial Instruments (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Financial Assets:
Loans receivable, Carrying Amount	$ 307,377,000		$ 895,665,000
Available-for-sale equity investments, Carrying Amount	1,037,000		1,384,000
Available-for-sale equity investments, Fair Value	1,037,000	[1]	1,384,000
Cash and cash equivalents, Carrying Amount	164,838,000		1,033,764,000	1,382,252,000	163,482,000
Cash and cash equivalents, Fair Value	164,838,000		1,033,764,000
Foreign currency forward contract, asset	7,669,000		0
Financial Liabilities:
Borrowings under unsecured line of credit arrangement	847,670,000		0
Borrowings under unsecured lines of credit arrangements, Fair Value	847,670,000		0
Senior unsecured notes, Carrying Amount	6,395,638,000		6,114,151,000
Senior unsecured notes, Fair Value	6,806,353,000		6,793,424,000
Secured debt, Carrying Amount	3,115,862,000		2,336,196,000
Secured debt, Fair Value	3,262,955,000		2,515,145,000
Interest rate swap agreements, Carrying Amount	68,000		264,000
Interest rate swap agreements, Fair Value	68,000		264,000
Foreign currency forward contact, liability	79,544,000		7,247,000
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	131,138,000		87,955,000
Loans receivable, Fair value	133,554,000		88,975,000
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	176,239,000		807,710,000
Loans receivable, Fair value	$ 182,153,000		$ 820,195,000

[1] (1) Unrealized gains or losses on equity investments are recorded in accumulated other comprehensive income (loss) at each measurement date.