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Fair Value Of Financial Instruments (Narrative) (Detail) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | $ 12 | $ 11
Percentage of level 3 assets at fair value in total assets measured at fair value value | 27.00% | 21.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value | 40.00% | 37.00%

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