Investments (Securities Pledged as Collateral Related to Repurchase Reverse Repurchase Program) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	$ 912	$ 732
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	1,568	1,333
Obligation to Return Securities Received as Collateral	2,480	2,065
Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	343	135
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	577	553
Debt Security, Government, Non-US [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	18	18
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	99	106
US States and Political Subdivisions Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	6	11
Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	209	229
Commercial Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	188	221
Residential mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	52	52
JAPAN
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	536	568
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	382	158
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	9	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	61	14
Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Obligation to Return Securities Received as Collateral	0	0
Maturity Overnight [Member] | Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | Debt Security, Government, Non-US [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
Maturity Overnight [Member] | Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | Commercial Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | Residential mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | JAPAN
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity Overnight [Member] | US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity up to 30 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	75	88
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	64	0
Obligation to Return Securities Received as Collateral	139	88
Maturity up to 30 days [Member] | Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	75	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	39	0
Maturity up to 30 days [Member] | Debt Security, Government, Non-US [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity up to 30 days [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
Maturity up to 30 days [Member] | Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity up to 30 days [Member] | Commercial Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	24	0
Maturity up to 30 days [Member] | Residential mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	1	0
Maturity up to 30 days [Member] | JAPAN
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	88
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity up to 30 days [Member] | US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	73	107
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	116	0
Obligation to Return Securities Received as Collateral	189	107
Maturity 30 to 90 Days [Member] | Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	66	94
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity 30 to 90 Days [Member] | Debt Security, Government, Non-US [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	7	13
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity 30 to 90 Days [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
Maturity 30 to 90 Days [Member] | Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	52	0
Maturity 30 to 90 Days [Member] | Commercial Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	56	0
Maturity 30 to 90 Days [Member] | Residential mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	8	0
Maturity 30 to 90 Days [Member] | JAPAN
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity 30 to 90 Days [Member] | US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	0	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	0	0
Maturity over 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	764	537
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	1,388	1,333
Obligation to Return Securities Received as Collateral	2,152	1,870
Maturity over 90 days [Member] | Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	202	41
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	538	553
Maturity over 90 days [Member] | Debt Security, Government, Non-US [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	11	5
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	99	106
Maturity over 90 days [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	6	11
Maturity over 90 days [Member] | Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	157	229
Maturity over 90 days [Member] | Commercial Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	108	221
Maturity over 90 days [Member] | Residential mortgage-backed securities
Assets Sold under Agreements to Repurchase [Line Items]
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	43	52
Maturity over 90 days [Member] | JAPAN
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	536	480
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	382	158
Maturity over 90 days [Member] | US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Fair Value of Collateral	9	0
FinancialAssetsSoldUnderAgreementsToRepurchaseGrossIncludingNotSubjectToMasterNettingArrangement	$ 61	$ 14