Disclosure about Fair Value of Financial Instruments - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial assets:
Equity securities	$ 11,860
Restricted cash	91,052	$ 100,753	[1]
Financial liabilities:
Unsecured revolving credit facility and commercial paper note program	1,869,188
Carrying Amount
Financial assets:
Equity securities	11,860	11,286
Cash and cash equivalents	268,666	215,376
Restricted cash	91,052	100,753
Foreign currency forward contracts, interest rate swaps and cross currency swaps	91,290	94,729
Financial liabilities:
Unsecured revolving credit facility and commercial paper note program		1,147,000
Senior unsecured notes	10,606,106	9,603,299
Secured debt	2,675,507	2,476,177
Foreign currency forward contracts, interest rate swaps and cross currency swaps	32,249	71,109
Redeemable OP unitholder interests	121,476	103,071
Carrying Amount | Mortgage loans receivable
Financial assets:
Loans receivable	264,398	249,071
Carrying Amount | Other real estate loans receivable
Financial assets:
Loans receivable	104,596	81,268
Fair Value
Financial assets:
Equity securities	11,860	11,286
Cash and cash equivalents	268,666	215,376
Restricted cash	91,052	100,753
Foreign currency forward contracts, interest rate swaps and cross currency swaps	91,290	94,729
Financial liabilities:
Unsecured revolving credit facility and commercial paper note program	1,869,188	1,147,000
Senior unsecured notes	11,026,259	10,043,797
Secured debt	2,737,838	2,499,130
Foreign currency forward contracts, interest rate swaps and cross currency swaps	32,249	71,109
Redeemable OP unitholder interests	121,476	103,071
Fair Value | Mortgage loans receivable
Financial assets:
Loans receivable	274,116	257,337
Fair Value | Other real estate loans receivable
Financial assets:
Loans receivable	$ 105,706	$ 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.