Investments - Gross Unrealized Appreciation (Depreciation) on Debt Securities by Type of Issuer (Details) $ in Millions	Sep. 30, 2025 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,654
Allowance for Credit Loss	(118)
Unrealized Appreciation	199
Unrealized Depreciation	(384)
Fair Value	8,351
Federal government and agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	214
Allowance for Credit Loss	0
Unrealized Appreciation	16
Unrealized Depreciation	(4)
Fair Value	226
State and local government
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24
Allowance for Credit Loss	0
Unrealized Appreciation	1
Unrealized Depreciation	0
Fair Value	25
Foreign government
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	407
Allowance for Credit Loss	0
Unrealized Appreciation	12
Unrealized Depreciation	(7)
Fair Value	412
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,738
Allowance for Credit Loss	(118)
Unrealized Appreciation	168
Unrealized Depreciation	(345)
Fair Value	7,443
Mortgage and other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	271
Allowance for Credit Loss	0
Unrealized Appreciation	2
Unrealized Depreciation	(28)
Fair Value	$ 245

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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