Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
FG VIEs’ Assets
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 150	$ 147
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	0	3
Other comprehensive income (loss)	0	0
Purchases	0	0
Sales		0
Settlements	(4)	(5)
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	146	145
Change in unrealized gains/(losses) related to financial instruments held	0	2
Consolidated Investment Vehicles
Fair Value Level 3 Rollforward
Fair value at beginning of period	167	99
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	4	19
Other comprehensive income (loss)	0	0
Purchases	0	0
Sales		0
Settlements	(165)	0
Deconsolidations	(6)
Transfers into Level 3	0
Fair value at end of period	0	118
Change in unrealized gains/(losses) related to financial instruments held	0	19
Other
Fair Value Level 3 Rollforward
Fair value at beginning of period	24	5
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	6	2
Other comprehensive income (loss)	0	(1)
Purchases	0	0
Sales		0
Settlements	0	0
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	30	6
Change in unrealized gains/(losses) related to financial instruments held	6	2
Other \| Other Comprehensive Income (Loss)
Total pre-tax realized and unrealized gains (losses) recorded in:
Change in unrealized gains/(losses) related to financial instruments held	0	(1)
Financial Guaranty Variable Interest Entities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at start of period	(156)	(164)
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	1	(2)
Other comprehensive income (loss)	0	(1)
Settlements	3	4
Fair value at end of period	(152)	(163)
Change in unrealized gains/(losses) included in earnings related to financial instruments	1	(2)
Change in unrealized gains/(losses) included in OCI related to financial instruments	0	(1)
Obligations of state and political subdivisions \| Fixed-maturity securities, available-for-sale:
Fair Value Level 3 Rollforward
Fair value at beginning of period	10	10
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	0	0
Other comprehensive income (loss)	0	0
Purchases	0	0
Sales		0
Settlements	0	0
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	10	10
Obligations of state and political subdivisions \| Fixed-maturity securities, available-for-sale: \| Other Comprehensive Income (Loss)
Total pre-tax realized and unrealized gains (losses) recorded in:
Change in unrealized gains/(losses) related to financial instruments held	0	6
Corporate securities \| Fixed-maturity securities, available-for-sale:
Fair Value Level 3 Rollforward
Fair value at beginning of period	168	0
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	1	0
Other comprehensive income (loss)	(7)	0
Purchases	0	5
Sales		0
Settlements	0	0
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	162	5
Corporate securities \| Fixed-maturity securities, available-for-sale: \| Other Comprehensive Income (Loss)
Total pre-tax realized and unrealized gains (losses) recorded in:
Change in unrealized gains/(losses) related to financial instruments held	(7)	0
RMBS \| Fixed-maturity securities, available-for-sale:
Fair Value Level 3 Rollforward
Fair value at beginning of period	144	145
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	2	3
Other comprehensive income (loss)	(1)	2
Purchases	0	0
Sales		0
Settlements	(5)	(6)
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	140	144
RMBS \| Fixed-maturity securities, available-for-sale: \| Other Comprehensive Income (Loss)
Total pre-tax realized and unrealized gains (losses) recorded in:
Change in unrealized gains/(losses) related to financial instruments held	(1)	(53)
Asset-backed securities \| Fixed-maturity securities, available-for-sale:
Fair Value Level 3 Rollforward
Fair value at beginning of period	511	1,031
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	(4)	7
Other comprehensive income (loss)	(46)	1
Purchases	14	0
Sales		(9)
Settlements	(46)	(47)
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	429	983
Asset-backed securities \| Fixed-maturity securities, available-for-sale: \| Other Comprehensive Income (Loss)
Total pre-tax realized and unrealized gains (losses) recorded in:
Change in unrealized gains/(losses) related to financial instruments held	(46)	(1)
Fixed-Maturity Securities, Trading \| Fixed-maturity securities, available-for-sale:
Fair Value Level 3 Rollforward
Fair value at beginning of period	4	5
Assured Life Re Acquisition	0
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	0	0
Other comprehensive income (loss)	0	0
Purchases	0	0
Sales		0
Settlements	(1)	0
Deconsolidations	0
Transfers into Level 3	5
Fair value at end of period	8	5
Change in unrealized gains/(losses) related to financial instruments held	0	(1)
Funds Withheld
Fair Value Level 3 Rollforward
Fair value at beginning of period	0
Assured Life Re Acquisition	293
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	3
Other comprehensive income (loss)	0
Purchases	0
Settlements	0
Deconsolidations	0
Transfers into Level 3	0
Fair value at end of period	296
Change in unrealized gains/(losses) related to financial instruments held	3
Credit Risk Contract
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value at start of period	(15)	(29)
Total pre-tax realized and unrealized gains (losses) recorded in:
Net income (loss)	1	104
Other comprehensive income (loss)	0	0
Settlements	(2)	(105)
Fair value at end of period	(16)	(30)
Change in unrealized gains/(losses) included in earnings related to financial instruments	$ (1)	$ (1)

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Amount of unrealized gain (loss) recognized in income for derivative asset (liability) after deduction of derivative liability (asset), measured at fair value using unobservable input (level 3) and still held.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Amount of gain (loss) recognized in income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Amount of gain (loss) recognized in other comprehensive income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
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-Section 50
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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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