Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Summary of financial instruments
Bank line of credit and commercial paper	$ 129,590	$ 93,000
Senior unsecured notes	5,697,586	5,647,993
Mortgage debt	221,621	12,317
Carrying Value
Summary of financial instruments
Loans receivable, net	195,375	190,579
Marketable debt securities	20,355	19,756
Bank line of credit and commercial paper	129,590	93,000
Term loan	249,182	248,942
Senior unsecured notes	5,697,586	5,647,993
Mortgage debt	221,621	12,317
Interest-rate swap liabilities	81	553
Carrying Value \| Discontinued Operations
Summary of financial instruments
Mortgage debt	319,000	297,000
Fair Value \| Level 1
Summary of financial instruments
Senior unsecured notes	6,517,650	6,076,150
Fair Value \| Level 2
Summary of financial instruments
Loans receivable, net	201,228	190,579
Marketable debt securities	20,355	19,756
Bank line of credit and commercial paper	129,590	93,000
Term loan	249,182	248,942
Mortgage debt	221,181	12,201
Interest-rate swap liabilities	$ 81	$ 553

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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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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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the operating cycle if longer, excluding current portion.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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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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