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Fair Value Of Financial Instruments (Narrative) (Detail) (USD $) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012 | Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |
Percentage of insured credit derivatives, value under Direct Price Model and Dual-Default Model | 2.00% |  | 2.00% |  |
Percentage of sector-specific transactions valued using BET model | 8.00% |  | 8.00% |  |
Percentage of transactions valued using corporate spreads used | 50.00% |  | 50.00% |  |
Percentage of transactions valued using spread benchmarked from most relevant spread source | 42.00% |  | 42.00% |  |
Percentage of transactions valued using WARF-sourced and/or ratings sourced credit spreads | 76.00% |  | 76.00% |  |
Net insured derivative liability | $ 1,400,000,000 |  | $ 1,400,000,000 |  | $ 2,900,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | 548,000,000 |  | 548,000,000 |  | 4,400,000,000
Percentage of level 3 assets at fair value in total assets measured at fair value value | 29.00% |  | 29.00% |  | 29.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value | 57.00% |  | 57.00% |  | 73.00%
Transfers into Level 3 | 41,000,000 | 168,000,000 | 199,000,000 | 270,000,000 |
Transfers out of Level 3 | 34,000,000 | 44,000,000 | 132,000,000 | 99,000,000 |
Transfers into Level 2 | 34,000,000 | 44,000,000 | 132,000,000 | 99,000,000 |
Transfers out of Level 2 | 41,000,000 | 168,000,000 | 199,000,000 | 270,000,000 |
Transfers into or out of Level 1 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0
B E T Valuation Model [Member] |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |
Percentage of insured credit derivatives, fair value using the BET model | 98.00% |  | 98.00% |  |

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