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Fair Value Of Financial Instruments (Company's Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | $ 4,277 |  | $ 5,131 |
Fair value financial liabilities measured on recurring basis | 690 |  | 786 |
Money Market Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 154 |  | 67 | [1]
Medium-term Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 108 |  | 102 |
Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 55 |  | 58 |
Cash and Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 75 |  | 222 |
Fixed Income Funds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 51 | [1] | 75 |
Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 3,202 |  | 3,871 |
Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 888 |  | 1,118 |
Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 200 |  | 728 |
Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 10 |  | 9 |
Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,266 |  | 1,410 |
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 330 |  | 219 |
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 19 |  | 28 |
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 22 |  | 54 |
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 140 |  | 121 |
Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 327 |  | 184 |
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  | 2 |
Assets Of Consolidated V I Es [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 486 |  | 418 |
Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 14 |
Assets Of Consolidated V I Es [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 45 |  | 92 |
Assets Of Consolidated V I Es [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 16 |  | 34 |
Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 6 |  | 7 |
Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 10 |
Assets Of Consolidated V I Es [Member] | Loans Receivable and Other Instruments At Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Residential Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 136 |  | 172 |
Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 17 |
Assets Of Consolidated V I Es [Member] | Cash and Cash Equivalents [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 58 |
Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 18 |  | 14 |
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 9 |  | 35 |
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 132 |  | 157 |
Derivative Liabilities [Member] | Other Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 34 |  | 7 |
Other Liabilities [Member] | Other Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 4 |  | 5 |
Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 403 |  | 480 |
Fair Value Inputs Level 1 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,058 |  | 1,331 |
Fair Value Inputs Level 1 [Member] | Money Market Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 154 |  |  |
Fair Value Inputs Level 1 [Member] | Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 30 |  | 23 |
Fair Value Inputs Level 1 [Member] | Cash and Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 75 |  | 222 |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 791 |  | 1,028 |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 791 |  | 1,028 |
Fair Value Inputs Level 1 [Member] | Assets Of Consolidated V I Es [Member] | Cash and Cash Equivalents [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 58 |
Fair Value Inputs Level 2 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2,519 |  | 3,021 |
Fair value financial liabilities measured on recurring basis | 190 |  | 273 |
Fair Value Inputs Level 2 [Member] | Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 25 |  | 35 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2,410 |  | 2,833 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 97 |  | 90 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | State and municipal bonds [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 200 |  | 728 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 10 |  | 9 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Corporate Obligations [Member] | Other Fixed Maturity Investments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,266 |  | 1,410 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 330 |  | 219 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 19 |  | 28 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 22 |  | 47 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 140 |  | 121 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 326 |  | 181 |
Fair Value Inputs Level 2 [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  | 2 |
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 9 |
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 45 |  | 92 |
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 16 |  | 34 |
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 6 |  | 6 |
Fair Value Inputs Level 2 [Member] | Assets Of Consolidated V I Es [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 10 |
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 2 |  | 2 |
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 132 |  | 157 |
Fair Value Inputs Level 2 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 56 |  | 114 |
Fair Value Inputs Level 3 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 649 |  | 637 |
Fair value financial liabilities measured on recurring basis | 500 |  | 513 |
Fair Value Inputs Level 3 [Member] | Medium-term Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 108 |  | 102 |
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  | 10 |
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 7 |
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  | 3 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 486 |  | 418 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Corporate Obligations [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 5 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 1 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Loans Receivable and Other Instruments At Fair Value [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Residential Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 136 |  | 172 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Currency Derivatives [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 17 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Other [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 18 |  | 14 |
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 7 |  | 33 |
Fair Value Inputs Level 3 [Member] | Derivative Liabilities [Member] | Other Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 34 |  | 7 |
Fair Value Inputs Level 3 [Member] | Other Liabilities [Member] | Other Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 4 |  | 5 |
Fair Value Inputs Level 3 [Member] | Liabilities Of Consolidated Vies [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | $ 347 |  | $ 366 |
[1] | Investment that was measured at fair value by applying the net asset value per share practical expedient, and was required not to be classified in the fair value hierarchy.

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