Disclosure about Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016	Sep. 30, 2016		Dec. 31, 2015
Financial Assets:
Loans receivable, Carrying Amount	$ 496,850		$ 622,628
Loans receivable, Fair value	413,221		521,773
Available-for-sale equity investments, Carrying Amount	7,615		27,899
Available-for-sale equity investments, Fair Value	7,615	[1]	27,899
Cash and cash equivalents, Carrying Amount	236,247		419,378	$ 428,617		$ 360,908
Cash and cash equivalents, Fair Value	236,247		419,378
Foreign Currency Forward Contracts Carrying Value	13,682		135,561
Foreign Currency Forward Contracts Fair Value	13,682		135,561
Financial Liabilities:
Borrowings under primary unsecured credit facilities, Carrying Value	420,000	[2]	645,000	$ 1,350,000	[2]
Borrowings under primary unsecured credit facilities, Fair Value	420,000		645,000
Senior unsecured notes, Carrying Amount	8,315,395		8,161,619
Senior unsecured notes, Fair Value	9,186,768		8,879,176
Secured debt, Carrying Amount	2,713,513		3,477,699
Secured debt, Fair Value	2,750,467		3,558,378
Foreign Currency Forward Contracts Carrying Value	55,264		4,342
Foreign Currency Forward Contracts, Fair Value	55,264		4,342
Redeemable OP unitholder interests carrying value	104,363		110,502
Redeemable OP unitholder interests fair value	104,363		110,502
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	377,283		485,735
Loans receivable, Fair value	413,221		521,773
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	119,567		136,893
Loans receivable, Fair value	$ 124,408		$ 138,050

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

[2] As of September 30, 2017, letters of credit in the aggregate amount of $25,576,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.