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Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total premiums earned	$ 202	$ 304	$ 385
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	169	238	310
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	27	30
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	4	4
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	3	3
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	25	26	29
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	2	3
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 3	$ 4	$ 6

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- Definition

Premiums Earned Net Financial Guarantee Insurance Contracts And Insured Derivatives

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No definition available.

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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.

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No definition available.

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