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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) segments $ / shares shares | Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |
Number of operating segments | segments |  | 3 |
Warrants exercised | shares |  | 11,850,000 |
Warrant exercise price | $ / shares |  | $ 9.59 |
Warrants outstanding | shares |  | 0 |
Treasury shares issued (in shares) | shares |  | 1,300,000 |
Zohar [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Aggregate claim amount |  | $ 919 |
Puerto Rico [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Claims payments |  | $ 276 |
SubsequentEvent [Member] | Puerto Rico [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Claims payments | $ 65 |  |
Assured Guaranty Corp [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Payment for sale of subsidiary |  |  | $ 23
Assured Guaranty Corp [Member] | Zohar II [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Outstanding bonds |  |  | $ 347
MBIA UK [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Sale effective date |  |  | Jan. 10, 2017
Gain (loss) on adjusting MBIA UK to its fair value less costs to sell |  |  | $ 5

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