Borrowings - Summary of Outstanding Debt (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Unsecured Debt	$ 3,879,526	$ 4,053,302
Total Outstanding Debt	$ 4,539,989
Debt Instrument, Interest Rate, Effective Percentage	3.80%
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	$ 3,879,526
Debt Instrument, Interest Rate, Effective Percentage	3.70%
Unsecured Debt [Member] \| Fixed Rate Debt Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 2,942,000
Debt Instrument, Interest Rate, Effective Percentage	4.00%
Contract Maturity	Nov. 26, 2025
Unsecured Debt [Member] \| Commercial Paper Notes [Member] \| Variable Rate Commercial Paper Program [Member]
Debt Instrument [Line Items]
Unsecured Debt	$ 367,000
Debt Instrument, Interest Rate, Effective Percentage	2.70%
Contract Maturity	Jul. 16, 2019
Unsecured Debt [Member] \| Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 300,000
Debt Instrument, Interest Rate, Effective Percentage	2.30%
Contract Maturity	Mar. 01, 2022
Unsecured Debt [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Unsecured Debt	$ 300,000
Debt Instrument, Interest Rate, Effective Percentage	3.40%
Contract Maturity	Aug. 29, 2020
Unsecured Debt [Member] \| $1 billion unsecured revolving credit facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	$ 0
Contract Maturity	May 31, 2023
Unsecured Debt [Member] \| $1 billion unsecured revolving credit facility [Member] \| Fair Market Value Adjustment and Debt Issuance Cost [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ (29,474)
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	$ 660,463
Debt Instrument, Interest Rate, Effective Percentage	4.50%
Secured Debt [Member] \| Fixed Rate Debt Senior Notes [Member]
Debt Instrument [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	$ 663,764
Debt Instrument, Interest Rate, Effective Percentage	4.50%
Contract Maturity	May 17, 2036
Secured Debt [Member] \| Fair Market Value Adjustment and Debt Issuance Cost [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	$ (3,301)

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount of investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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