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Fair Value Measurement (Narrative) (Detail) (USD $) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 | Jun. 30, 2011 | Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Percentage of sector-specific transactions valued using BET model | 9.00% |  | 9.00% |  |
Percentage of transactions valued using corporate spreads used | 47.00% |  | 47.00% |  |
Percentage of transactions valued using spread benchmarked from most relevant spread source | 44.00% |  | 44.00% |  |
Percentage of transactions valued using WARF-sourced and/or ratings sourced credit spreads | 76.00% |  | 76.00% |  |
Net insured derivative liability | $ 3,300,000,000 |  | $ 3,300,000,000 |  | $ 4,800,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | 4,300,000,000 |  | 4,300,000,000 |  | 5,700,000,000
Percentage of level 3 assets at fair in total assets measured at fair value value |  |  | 28.00% |  | 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 | 72,000,000 | 166,000,000 | 103,000,000 | 208,000,000 |
Transfers out of Level 3 | 15,000,000 | 28,000,000 | 55,000,000 | 264,000,000 |
Transfers into Level 2 | 15,000,000 | 28,000,000 | 55,000,000 | 264,000,000 |
Transfers out of Level 2 | 72,000,000 | 166,000,000 | 103,000,000 | 208,000,000 |
Transfers into or out of Level 1 | 0 |  | 0 |  |
Net unrealized loss related to transfer into Level 3 | 8,000,000 | 2,000,000 | 17,000,000 | 1,000,000 |
Net unrealized losses on securities transferred out of Level 3 | 2,000,000 |  | 5,000,000 |  |
Net unrealized loss related to transfers out of Level 3 |  | $ 8,000,000 |  | $ 22,000,000 |
BET 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 | 79.00% |  | 79.00% |  |
Direct Price Model [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Percentage of insured credit derivatives, value under Direct Price Model | 21.00% |  | 21.00% |  |

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