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Fair Value Of Financial Instruments (Narrative) (Detail) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Net insured derivative liability | $ 2,900,000,000 | $ 4,800,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | 4,400,000,000 | 5,700,000,000
Interest rate on the surplus notes | 14.00% |
Percentage of level 3 assets at fair value in total assets measured at fair value | 29.00% | 29.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value | 73.00% | 77.00%
Transfers into Level 3 | 293,000,000 | 456,000,000
Transfers out of Level 3 | 157,000,000 | 391,000,000
Transfers into Level 2 | 157,000,000 | 391,000,000
Transfers out of Level 2 | 293,000,000 | 456,000,000
Transfers into or out of Level 1 | $ 0 | $ 0
Binomial Expansion Technique [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of insured credit derivatives | 82.00% |
Percentage of sector specific transactions | 9.00% |
Corporate Spreads [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of sector specific transactions | 46.00% |
Spreads Benchmarked Relevant Spread Source [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of sector specific transactions | 45.00% |
WARF-Sourced [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of sector specific transactions | 79.00% |
Direct Price Model [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of insured credit derivatives | 18.00% |

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