Disclosure about Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017		Dec. 31, 2016	Mar. 31, 2016		Dec. 31, 2015
Financial Assets:
Loans receivable, Carrying Amount	$ 574,080		$ 622,628
Loans receivable, Fair value	465,548		521,773
Available-for-sale equity investments, Carrying Amount	17,330		27,899
Available-for-sale equity investments, Fair Value	17,330	[1]	27,899
Cash and cash equivalents, Carrying Amount	380,360		419,378	$ 355,949		$ 360,908
Cash and cash equivalents, Fair Value	380,360		419,378
Foreign Currency Forward Contracts Carrying Value	110,439		135,561
Foreign Currency Forward Contracts Fair Value	110,439		135,561
Financial Liabilities:
Borrowings under primary unsecured credit facilities, Carrying Value	522,000	[2]	645,000	$ 645,000	[2]
Borrowings under primary unsecured credit facilities, Fair Value	522,000		645,000
Senior unsecured notes, Carrying Amount	8,188,928		8,161,619
Senior unsecured notes, Fair Value	8,965,318		8,879,176
Secured debt, Carrying Amount	2,669,787		3,477,699
Secured debt, Fair Value	2,728,171		3,558,378
Foreign Currency Forward Contracts Carrying Value	9,840		4,342
Foreign Currency Forward Contracts, Fair Value	9,840		4,342
Redeemable OP unitholder interests carrying value	116,917		110,502
Redeemable OP unitholder interests fair value	116,917		110,502
Mortgage Loans on Real Estate [Member]
Financial Assets:
Loans receivable, Carrying Amount	430,607		485,735
Loans receivable, Fair value	465,548		521,773
Other Real Estate Loans Receivable [Member]
Financial Assets:
Loans receivable, Carrying Amount	143,473		136,893
Loans receivable, Fair value	$ 149,151		$ 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 March 31, 2017, letters of credit in the aggregate amount of $32,456,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.