| **Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions** | Sep. 30, 2020 | Dec. 31, 2019 |
|---------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Finance receivable                                                                    | $ 86          | $ 77          |
| Loss and loss expense reserves                                                        | 2,900         | 2,835         |
| Life Policy Loans                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Finance receivable                                                                    | 33            | 32            |
| Investment contract reserves:                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Loss and loss expense reserves                                                        | 1,562         | 1,559         |
| Investment contract reserves: | Deferred annuities                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Loss and loss expense reserves                                                        | 760           | 760           |
| Investment contract reserves: | Structured Settlement Annuity                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Loss and loss expense reserves                                                        | 147           | 151           |
| Fair Value, Inputs, Level 1                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**   |          |          |
| Cash and Cash Equivalents, Fair Value Disclosure                                      | $ 914         | $ 767         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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| - Definition
Amount of accrued obligation to policyholders that relates to insured events under long-term life insurance contracts.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.
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