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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, unless otherwise specified | Sep. 30, 2012 | Jun. 30, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Dec. 31, 2010
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | $ 12,267 |  |  | $ 16,206 |  |  |
Fair value financial liabilities measured on recurring basis | 7,324 |  |  | 10,946 |  |  |
Cash And Cash Equivalents [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 298 |  |  | 473 |  |  |
Variable Interest Entity [Member] | Cash [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 143 |  |  | 160 |  |  |
Variable Interest Entity [Member] | Loans Receivable [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,892 |  |  | 2,046 |  |  |
Fair value financial assets measured on recurring basis | 1,892 |  |  |  |  |  |
Variable Interest Entity [Member] | Loan Repurchase Commitments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,051 |  |  | 1,077 |  |  |
Fair value financial assets measured on recurring basis | 1,051 |  |  |  |  |  |
Money Market Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 593 |  |  | 912 |  |  |
Perpetual Preferred Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 31 |  |  | 107 |  |  |
Medium-Term Notes [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 168 |  |  | 165 |  |  |
Total Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 6,683 |  |  | 8,682 |  |  |
Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 6,028 |  |  | 7,628 |  |  |
Fixed-Maturity Investments [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 2,196 |  |  | 3,316 |  |  |
Fixed-Maturity Investments [Member] | Other Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 10 |  |  | 15 |  |  |
Fixed-Maturity Investments [Member] | Corporate Obligations [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 302 |  |  | 239 |  |  |
Fixed-Maturity Investments [Member] | Residential Mortgage-Backed Agency [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 3 |  |  |
Fixed-Maturity Investments [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 860 |  |  | 1,458 |  |  |
Fixed-Maturity Investments [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 422 |  |  | 581 |  |  |
Fixed-Maturity Investments [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 364 |  |  | 533 |  |  |
Fixed-Maturity Investments [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 192 |  |  | 303 |  |  |
Fixed-Maturity Investments [Member] | State And Municipal Bonds [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 41 |  |  |  |  |  |
Fixed-Maturity Investments [Member] | Money Market Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 15 |  |  | 199 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 5,070 |  |  | 6,448 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | U.S. Treasury And Government Agency [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 961 |  |  | 1,141 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Foreign Governments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 341 |  |  | 350 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,228 |  |  | 1,739 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,073 |  |  | 1,284 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 176 |  |  | 367 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 49 |  |  | 58 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 94 |  |  | 138 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 189 |  |  | 447 |  |  |
Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 959 |  |  | 924 |  |  |
Fixed-Maturity Investments [Member] | Tax-Exempt Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 948 |  |  | 1,165 |  |  |
Other Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 31 |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 35 |  |  |
Derivative Assets [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 4 |  |  | 2 |  |  |
Derivative Assets [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  |  |  |  |  |
Derivative Assets [Member] | Credit Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 1 |  |  |
Derivative Assets [Member] | Credit Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 447 |  |  |
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 3 |  |  | 94 |  |  |
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 3 |  |  |
Derivative Assets [Member] | Other [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | (93) |  |  |
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 3,332 |  |  | 4,808 |  |  |
Derivative Liabilities [Member] | Credit Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  |  | 527 |  |  |
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | (2) |  |  | 445 |  |  |
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 157 |  |  | 281 |  |  |
Derivative Liabilities [Member] | Other [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  |  | (93) |  |  |
Derivative Liabilities [Member] | Currency Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 2 |  |  | 4 |  |  |
Derivative Liabilities [Member] | Currency Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 23 |  |  | 17 |  |  |
Other Liabilities [Member] | Warrants [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 18 |  |  | 38 |  |  |
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 3,626 |  |  | 4,754 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 2,187 |  |  | 3,085 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Cash And Cash Equivalents [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 298 |  |  | 473 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Variable Interest Entity [Member] | Cash [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 143 |  |  | 160 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Money Market Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 593 |  |  | 912 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Total Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,731 |  |  | 2,253 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,118 |  |  | 1,316 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 15 |  |  | 199 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Money Market Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 15 |  |  | 199 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,118 |  |  | 1,316 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | U.S. Treasury And Government Agency [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 860 |  |  | 1,038 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Foreign Governments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 257 |  |  | 277 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  |  | 1 |  |  |
Quoted Prices In Active Markets for Identical Assets (Level 1) [Member] | Other Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 20 |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 25 |  |  |
Significant Other Observable Inputs (Level 2) [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 6,534 |  |  | 8,576 |  |  |
Fair value financial liabilities measured on recurring basis | 2,236 |  |  | 2,651 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Perpetual Preferred Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 28 |  |  | 106 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Total Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 4,601 |  |  | 5,746 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 4,573 |  |  | 5,640 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,838 |  |  | 2,735 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Other Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 10 |  |  | 15 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Corporate Obligations [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 219 |  |  | 170 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Residential Mortgage-Backed Agency [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 3 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 852 |  |  | 1,437 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 409 |  |  | 559 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 191 |  |  | 330 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 126 |  |  | 236 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | State And Municipal Bonds [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 41 |  |  |  |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 3,638 |  |  | 4,488 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | U.S. Treasury And Government Agency [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 101 |  |  | 103 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Foreign Governments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 72 |  |  | 62 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,136 |  |  | 1,531 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,073 |  |  | 1,276 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 169 |  |  | 350 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 19 |  |  | 34 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 64 |  |  | 78 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 116 |  |  | 130 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 888 |  |  | 924 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Fixed-Maturity Investments [Member] | Tax-Exempt Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 925 |  |  | 1,137 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Assets [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 95 |  |  | 92 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Assets [Member] | Credit Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 1 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 95 |  |  | 91 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 3 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 14 |  |  | 18 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 305 |  |  | 445 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 157 |  |  | 281 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Derivative Liabilities [Member] | Currency Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 1 |  |  | 4 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Other Liabilities [Member] | Warrants [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 18 |  |  | 38 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 1,741 |  |  | 1,865 |  |  |
Significant Unobservable Inputs (Level 3) [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 3,642 |  |  | 4,638 |  |  |
Fair value financial assets measured on recurring basis | 3,637 |  | 3,544 | 4,188 | 4,309 | 4,528 | 4,467
Fair value financial liabilities measured on recurring basis | 5,399 |  |  | 8,388 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Variable Interest Entity [Member] | Loans Receivable [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,892 |  |  | 2,046 |  |  |
Fair value financial assets measured on recurring basis | 1,892 |  | 1,903 | 2,046 | 2,218 | 2,320 | 2,183
Significant Unobservable Inputs (Level 3) [Member] | Variable Interest Entity [Member] | Loan Repurchase Commitments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1,051 |  |  | 1,077 |  |  |
Fair value financial assets measured on recurring basis | 1,051 |  | 1,032 | 1,077 | 938 | 905 | 835
Significant Unobservable Inputs (Level 3) [Member] | Foreign Governments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 12 |  | 12 | 11 | 11 | 13 | 11
Significant Unobservable Inputs (Level 3) [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 91 |  | 98 | 207 | 342 | 294 | 246
Significant Unobservable Inputs (Level 3) [Member] | Corporate Obligations [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 83 |  | 95 | 69 | 63 | 62 | 82
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-Backed Agency [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 4 | 8 | 1 |  | 41
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-Backed Non-Agency [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 7 |  | 33 | 17 | 27 | 28 | 48
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-Backed Non-Agency [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 8 |  | 10 | 21 | 15 | 17 | 40
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-Backed [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 30 |  | 27 | 24 | 32 | 50 | 41
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-Backed [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 13 |  | 12 | 22 | 17 | 27 | 23
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Debt Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 30 |  | 29 | 60 | 67 | 162 | 192
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Debt Obligations [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 173 |  | 140 | 203 | 206 | 201 | 245
Significant Unobservable Inputs (Level 3) [Member] | Other Asset-Backed [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 73 |  | 69 | 317 | 270 | 331 | 349
Significant Unobservable Inputs (Level 3) [Member] | Other Asset-Backed [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 66 |  | 42 | 67 | 71 | 73 | 81
Significant Unobservable Inputs (Level 3) [Member] | Perpetual Preferred Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 3 | [1] |  | 1 |  |  |
Fair value financial assets measured on recurring basis | 3 |  | 3 | 1 | 1 |  |
Significant Unobservable Inputs (Level 3) [Member] | Medium-Term Notes [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 168 | [1] |  | 165 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Total Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 351 |  |  | 683 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 337 |  |  | 672 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 343 |  |  | 382 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Corporate Obligations [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 83 | [1] |  | 69 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 8 | [1] |  | 21 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 13 | [1] |  | 22 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 173 | [1] |  | 203 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 66 | [1] |  | 67 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 314 |  |  | 644 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Foreign Governments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 12 | [1] |  | 11 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Corporate Obligations [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 91 | [1] |  | 207 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 8 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Residential Mortgage-Backed Non-Agency [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 7 | [1] |  | 17 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Commercial Mortgage-Backed [Member] | Mortgage-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 30 | [1] |  | 24 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Collateralized Debt Obligations [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 30 | [1] |  | 60 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | Other Asset-Backed [Member] | Asset-Backed Securities [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 73 | [1] |  | 317 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Taxable Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 71 | [1] |  |  |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Fixed-Maturity Investments [Member] | Tax-Exempt Bonds [Member] | State And Municipal Bonds [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 23 | [1] |  | 28 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Other Investments [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 11 | [1] |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 10 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Assets [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 5 |  |  | 3 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Assets [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 1 |  |  |  |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Assets [Member] | Credit Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | 447 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | 4 | [1] |  | 3 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 3,318 |  |  | 4,790 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  |  | 527 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 4 | [1] |  |  |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Liabilities [Member] | Currency Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 1 | [1] |  |  |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Derivative Liabilities [Member] | Currency Derivatives [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 23 | [1] |  | 17 |  |  |
Significant Unobservable Inputs (Level 3) [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | 1,885 |  |  | 2,889 |  |  |
Counterparty and Cash Collateral Netting [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | (96) |  |  | (93) |  |  |
Fair value financial liabilities measured on recurring basis | (311) |  |  | (93) |  |  |
Counterparty and Cash Collateral Netting [Member] | Derivative Assets [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | (96) |  |  | (93) |  |  |
Counterparty and Cash Collateral Netting [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis | (96) |  |  |  |  |  |
Counterparty and Cash Collateral Netting [Member] | Derivative Assets [Member] | Other [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial assets measured on recurring basis |  |  |  | (93) |  |  |
Counterparty and Cash Collateral Netting [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis | (311) |  |  |  |  |  |
Counterparty and Cash Collateral Netting [Member] | Derivative Liabilities [Member] | Other [Member] | Non-Insured Derivatives [Member] |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  |  | $ (93) |  |  |
[1] | 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.

X
- Definition Fair Value Liabilities Measured On Recurring Basis + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Details
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Namespace Prefix: | us-gaap_
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- Definition This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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Period Type: | instant

X
- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16383099&loc=d3e24584-111560 + Details
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant