Disclosure about Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Assets:
Loans receivable, Carrying Amount		$ 495,871	$ 622,628
Available-for-sale equity investments, Carrying Amount		7,269	27,899
Available-for-sale equity investments, Fair Value		7,269	27,899
Cash and cash equivalents, Carrying Amount		243,777	419,378	$ 422,690	$ 553,423
Cash and cash equivalents, Fair Value		243,777	419,378
Restricted cash, Carrying Value		65,526	187,842
Restricted cash, Fair Value		65,526	187,842
Foreign Currency Forward Contracts Carrying Value		15,604	135,561
Foreign Currency Forward Contracts Fair Value		15,604	135,561
Financial Liabilities:
Borrowings under unsecured lines of credit arrangements	[1]	719,000	645,000	$ 835,000
Borrowings under unsecured lines of credit arrangements, Fair Value		719,000	645,000
Senior unsecured notes, Carrying Amount		8,331,722	8,161,619
Senior unsecured notes, Fair Value		9,168,432	8,879,176
Secured debt, Carrying Amount		2,608,976	3,477,699
Secured debt, Fair Value		2,641,997	3,558,378
Foreign Currency Forward Contracts Carrying Value		38,654	4,342
Foreign Currency Contracts, Liability, Fair Value Disclosure		38,654	4,342
Redeemable OP unitholder interests carrying value		97,476	110,502
Redeemable OP unitholder interests fair value		97,476	110,502
Mortgage Loans [Member]
Financial Assets:
Loans receivable, Carrying Amount		306,120	485,735
Loans receivable, Fair value		332,508	521,773
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount		121,379	136,893
Loans receivable, Fair value		$ 125,480	$ 138,050

[1] As of December 31, 2017, letters of credit in the aggregate amount of $22,365,000 have been issued, which reduce the available borrowing capacity on our primary unsecured revolving credit facility.