Disclosure about Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financial Assets:
Loans receivable, Carrying Amount	$ 449,467	$ 495,871
Equity securities, carrying amount	21,903	7,269
Equity securities, fair value disclosure	21,903	7,269
Cash and Cash Equivalents, at Carrying Value	215,120	243,777
Cash and cash equivalents, Fair Value	215,120	243,777
Restricted cash, carrying amount	57,263	65,526
Restricted Cash, Fair Value Disclosure	57,263	65,526
Foreign Currency Forward Contracts Carrying Value	104,729	15,604
Foreign Currency Forward Contracts Fair Value	104,729	15,604
Financial Liabilities:
Borrowings under primary unsecured credit facility, Carrying Value	540,000	719,000	$ 385,000
Borrowings under primary unsecured credit facilities, Fair Value	540,000	719,000
Senior unsecured notes, Carrying Amount	8,373,774	8,331,722
Senior unsecured notes, Fair Value	8,904,455	9,168,432
Secured debt, Carrying Amount	2,450,483	2,608,976
Secured debt, Fair Value	2,468,073	2,641,997
Foreign Currency Forward Contracts Carrying Value	53,876	38,654
Foreign Currency Forward Contracts, Fair Value	53,876	38,654
Redeemable OP unitholder interests carrying value	97,476	97,476
Redeemable OP unitholder interests fair value	97,476	97,476
Mortgage Loans on Real Estate
Financial Assets:
Loans receivable, Carrying Amount	274,226	306,120
Loans receivable, Fair value	282,406	332,508
Other Real Estate Loans Receivable
Financial Assets:
Loans receivable, Carrying Amount	106,869	121,379
Loans receivable, Fair value	$ 110,486	$ 125,480