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Fair Value Of Financial Instruments (Narrative) (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Net insured derivative liability | $ 64 | $ 85 | $ 64 | $ 85
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | $ 11 | $ 99 | $ 11 | $ 99
Percentage of level 3 assets at fair value in total assets measured at fair value value | 21.00% | 19.00% | 21.00% | 19.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value | 37.00% | 52.00% | 37.00% | 52.00%
Transfers into or out of Level 1 | $ 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 | 97.00% |  | 97.00% |
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 and Dual-Default Model | 3.00% |  | 3.00% |

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- Definition Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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