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Loss and Loss Adjustment Expense Reserves (Loss and LAE Activity) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Loss And Loss Adjustment Expense Reserves [Line Items]
Weighted average risk-free rates used to discount claim liability	4.19%		4.50%
Lae [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Losses and loss adjustment	$ 1	$ 2
Loss and loss adjustment expense reserves	$ 18		$ 19
Changes in Loss and LAE Reserves	one-year

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- Definition

Changes in Loss and LAE Reserves

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Loss And Loss Adjustment Expense Reserves [Line Items]

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- Definition

Weighted-average risk-free rate used to discount claim liability for financial guarantee insurance contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

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- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-7

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
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-Section 50
-Paragraph 3
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Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(13)(a)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
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-Publisher FASB
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