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%

Approximate percentage of level 3 assets at fair value in total assets measured at fair value.

No definition available.

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- Definition

Approximate percentage of level 3 liabilities at fair value in total liabilities measured at fair value.

+ References

No definition available.

+ Details

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- Definition

Impact Of Non Performance Risk On Insured Derivative Liability

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No definition available.

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- Definition

Percentage Of Insured Credit Derivatives Value Under Bet Model

+ References

No definition available.

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- Definition

Percentage of insured credit derivatives value under direct price model and dual default model

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No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis into and out of level one during the period.

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No definition available.

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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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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