Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Summary of financial instruments
Bank line of credit and commercial paper	$ 1,038,150	$ 129,590
Senior unsecured notes	4,255,697	5,697,586
Mortgage debt	219,959	221,621
Carrying Value
Summary of financial instruments
Loans receivable, net	740,142	195,375
Marketable debt securities	20,512	20,355
Bank line of credit and commercial paper	1,038,150	129,590
Term loan	249,243	249,182
Senior unsecured notes	4,255,697	5,697,586
Mortgage debt	219,959	221,621
Interest-rate swap liabilities	0	81
Carrying Value \| Discontinued Operations
Summary of financial instruments
Mortgage debt	278,000	319,000
Fair Value \| Level 1
Summary of financial instruments
Senior unsecured notes	4,664,075	6,517,650
Fair Value \| Level 2
Summary of financial instruments
Loans receivable, net	743,465	201,228
Marketable debt securities	20,512	20,355
Bank line of credit and commercial paper	1,038,150	129,590
Term loan	249,243	249,182
Mortgage debt	219,510	221,181
Interest-rate swap liabilities	$ 0	$ 81

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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