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Fair Value Of Financial Instruments (Company's Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | $ 7,420 | $ 7,564 |
Fair value financial liabilities measured on recurring basis |  | 1,570 | 1,784 |
Money Market Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 269 | 521 |
Medium Term Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  | 127 | 101 |
Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 47 | 35 |
Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 116 | 163 |
Fixed Income Funds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | [1] | 81 | 75 |
Other [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 17 |  |
Other Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  | 7 |  |
Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 4,627 | 4,999 |
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 |  | 817 | 937 |
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 |  | 1,113 | 1,440 |
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 |  | 8 | 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 |  | 1,511 | 1,334 |
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 |  | 699 | 868 |
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 |  | 36 | 45 |
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 |  | 51 | 43 |
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 |  | 72 | 22 |
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 |  | 320 | 301 |
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 |  | 3 | 3 |
Derivative Assets [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 |  | 13 | 19 |
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 |  | 406 | 404 |
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 |  | 20 | 27 |
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 |  | 111 | 149 |
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 |  | 39 | 52 |
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 |  | 9 | 8 |
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 |  | 10 | 19 |
Assets Of Consolidated V I Es [Member] | Loans Receivable 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 |  | 759 | 916 |
Assets Of Consolidated V I Es [Member] | Loans Receivable At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Corporate Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 873 | 150 |
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 |  | 20 | 24 |
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  | 76 | 66 |
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 |  | 204 | 213 |
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 |  | 4 | 20 |
Other Liabilities [Member] | Warrant [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  | 12 | 33 |
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 |  | 1,140 | 1,351 |
Fair Value Inputs Level 1 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 1,155 | 1,559 |
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 |  | 269 | 521 |
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 |  | 26 | 26 |
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 |  | 116 | 163 |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 724 | 825 |
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 |  | 724 | 825 |
Fair Value Inputs Level 1 [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 |  | 0 |  |
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 |  | 20 | 24 |
Fair Value Inputs Level 2 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 4,110 | 4,378 |
Fair value financial liabilities measured on recurring basis |  | 928 | 1,123 |
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 |  | 21 | 9 |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 3,898 | 4,113 |
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 |  | 93 | 112 |
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 |  | 1,113 | 1,440 |
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 |  | 8 | 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 |  | 1,511 | 1,332 |
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 |  | 699 | 868 |
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 |  | 36 | 45 |
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 |  | 51 | 43 |
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 |  | 72 | 7 |
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 |  | 315 | 257 |
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 |  | 3 | 3 |
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 |  | 20 | 27 |
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 |  | 111 | 149 |
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 |  | 39 | 52 |
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 |  | 8 | 7 |
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 |  | 10 | 18 |
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 |  | 204 | 213 |
Fair Value Inputs Level 2 [Member] | Other Liabilities [Member] | Warrant [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  | 12 | 33 |
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 |  | 710 | 875 |
Fair Value Inputs Level 3 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 2,074 | 1,552 |
Fair value financial liabilities measured on recurring basis |  | 642 | 661 |
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 | [2] | 127 | 101 |
Fair Value Inputs Level 3 [Member] | Other [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | [2] | 17 |  |
Fair Value Inputs Level 3 [Member] | Other Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | [2] | 7 |  |
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 5 | 61 |
Fair Value Inputs Level 3 [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 | [2] |  | 2 |
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 | [2] | 0 |  |
Fair Value Inputs Level 3 [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 | [2] |  | 15 |
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 | [2] | 5 | 44 |
Fair Value Inputs Level 3 [Member] | Derivative Assets [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 |  | 13 | 19 | [2]
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 |  | 406 | 404 |
Fair Value Inputs Level 3 [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 | [2] | 0 |  |
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 | [2] | 1 | 1 |
Fair Value Inputs Level 3 [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 | [2] |  | 1 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Loans Receivable 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 |  | 759 | 916 |
Fair Value Inputs Level 3 [Member] | Assets Of Consolidated V I Es [Member] | Loans Receivable At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Corporate Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  | 873 | 150 | [2]
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 |  | 74 | 64 |
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 |  | 4 | 20 |
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 |  | $ 430 | $ 476 |
[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.
[2] | Unobservable inputs are either not developed by the Company or do not significantly impact the overall fair values of the aggregate financial assets and liabilities.

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