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Business Segments (Summary Of Company's Segment Results) (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Operating Segments [Member] | U S Public Finance Insurance [Member] |  |
Segment Reporting Information [Line Items] |  |
Net investment income | $ 15 | $ 18
Operating Segments [Member] | Corporate Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Net investment income | 7 | 8
Operating Segments [Member] | International And Structured Finance Insurance [Member] |  |
Segment Reporting Information [Line Items] |  |
Net investment income | 3 | 3
Intersegment Eliminations [Member] |  |
Segment Reporting Information [Line Items] |  |
Net investment income | $ (7) | $ (6)

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