Disclosure about Fair Value of Financial Instruments - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Equity securities	$ 13,773
Restricted cash	158,312	$ 100,753	[1]
Carrying Amount
Financial assets:
Equity securities	13,773	11,286
Cash and cash equivalents	249,127	215,376
Restricted cash	158,312	100,753
Foreign currency forward contracts and cross currency swaps	53,078	94,729
Financial liabilities:
Unsecured revolving credit facility and unsecured commercial paper note program	419,293	1,147,000
Senior unsecured notes	9,632,013	9,603,299
Secured debt	2,660,190	2,476,177
Foreign currency forward contracts and cross currency swaps	85,687	71,109
Redeemable OP unitholder interests	115,218	103,071
Carrying Amount | Mortgage loans receivable
Financial assets:
Loans receivable	258,315	249,071
Carrying Amount | Other real estate loans receivable
Financial assets:
Loans receivable	92,770	81,268
Fair Value
Financial assets:
Equity securities	13,773	11,286
Cash and cash equivalents	249,127	215,376
Restricted cash	158,312	100,753
Foreign currency forward contracts and cross currency swaps	53,078	94,729
Financial liabilities:
Unsecured revolving credit facility and unsecured commercial paper note program	419,293	1,147,000
Senior unsecured notes	10,409,527	10,043,797
Secured debt	2,709,741	2,499,130
Foreign currency forward contracts and cross currency swaps	85,687	71,109
Redeemable OP unitholder interests	115,218	103,071
Fair Value | Mortgage loans receivable
Financial assets:
Loans receivable	267,143	257,337
Fair Value | Other real estate loans receivable
Financial assets:
Loans receivable	$ 93,767	$ 82,742

[1] The consolidated balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.