Investments - Gross Unrealized Appreciation (Depreciation) on Debt Securities by Type of Issuer (Details) $ in Millions	Mar. 31, 2025 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,737
Allowance for Credit Loss	(107)
Unrealized Appreciation	143
Unrealized Depreciation	(558)
Fair Value	8,215
Federal government and agency
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	211
Allowance for Credit Loss	0
Unrealized Appreciation	16
Unrealized Depreciation	(4)
Fair Value	223
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	359
Allowance for Credit Loss	0
Unrealized Appreciation	8
Unrealized Depreciation	(9)
Fair Value	358
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,786
Allowance for Credit Loss	(107)
Unrealized Appreciation	116
Unrealized Depreciation	(515)
Fair Value	7,280
Mortgage and other asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	357
Allowance for Credit Loss	0
Unrealized Appreciation	2
Unrealized Depreciation	(30)
Fair Value	$ 329

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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