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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, 2018 | Dec. 31, 2017
--- | --- | ---
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | $ 5,421 | $ 7,102
Fair value financial liabilities measured on recurring basis |  | 1,010 | 1,459
Money Market Securities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 147 | 180
Medium Term Notes [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial liabilities measured on recurring basis |  | 123 | 115
Perpetual Debt And Equity Securities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 64 | 63
Cash And Cash Equivalents [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 167 | 122
Fixed Income Funds [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis | [1] | 75 | 82
Fixed Maturities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 3,919 | 4,328
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 |  | 854 | 1,352
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 |  | 757 | 858
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 |  | 11 | 10
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 |  | 1,623 | 1,340
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 |  | 220 | 368
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 |  | 30 | 32
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 |  | 57 | 67
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 |  | 146 | 118
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 |  | 221 | 183
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 |  | 2 | 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 |  | 415 | 407
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 |  | 14 | 19
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 |  | 98 | 108
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 |  | 35 | 36
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 |  | 7 | 9
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 |  | 9 | 10
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 |  | 428 | 759
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 |  | 0 | 920
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 |  | 14 | 19
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 |  | 12 | 24
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 |  | 15 | 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 |  | 29 | 65
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 |  | 137 | 193
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 |  | 7 | 4
Other Liabilities [Member] | Warrant [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial liabilities measured on recurring basis |  |  | 6
Other Liabilities [Member] | Other Payable [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial liabilities measured on recurring basis |  | 5 | 7
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 |  | 709 | 1,069
Fair Value Inputs Level 1 [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 1,137 | 1,608
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 |  | 147 | 180
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 |  | 167 | 122
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 785 | 1,256
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 |  | 765 | 1,256
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 |  | 20 |
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 |  | 12 | 24
Fair Value Inputs Level 2 [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 3,320 | 3,272
Fair value financial liabilities measured on recurring basis |  | 466 | 864
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 |  | 38 | 37
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 3,123 | 3,058
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 |  | 89 | 96
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 |  | 757 | 858
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 |  | 11 | 10
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 |  | 1,603 | 1,338
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 |  | 220 | 368
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 |  | 30 | 32
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 |  | 50 | 60
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 |  | 146 | 118
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 |  | 217 | 178
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 |  | 2 | 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 |  | 9 | 19
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 |  | 98 | 108
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 |  | 35 | 30
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 | 8
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 |  | 9 | 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 |  | 137 | 193
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 |  |  | 6
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 |  | 327 | 663
Fair Value Inputs Level 3 [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 889 | 2,140
Fair value financial liabilities measured on recurring basis |  | 544 | 595
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] | 123 | 115
Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Fair value financial assets measured on recurring basis |  | 11 | 14
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] | 7 | 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 | [2] | 4 | 5
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 |  | 415 | 407
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 | [2] | 5 |
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] |  | 6
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] | 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 |  | 428 | 759
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 | [2] | 0 | 920
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 | [2] | 14 | 19
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 | [2] | 15 | 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 |  | 27 | 63
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 |  | 7 | 4
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 | [2] | 5 | 7
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 |  | $ 382 | $ 406
[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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