Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 45,632	$ 46,041
Other Short-term Investments	4,223	4,353
Equity Securities, FV-NI	488	492
ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	4,668	4,663
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	3,330	3,316
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	2,232	2,328
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	23,305	23,076
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	6,092	6,178
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	3,211	3,372
Other Short-term Investments	688	382
Equity Securities, FV-NI	123	106
Significant Unobservable Inputs (Level 3) | ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	163	167
Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	233	217
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	116	115
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	2,694	2,866
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	5	7
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | ABS | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 163	$ 167
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | ABS | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.69	4.34
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | ABS | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	9.41	8.43
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | ABS | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	5.68	5.30
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 233	$ 217
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.79	2.70
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	10.41	10.21
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Collateralized Loan Obligations [Member] | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	5.56	5.44
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | CMBS | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 116	$ 115
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | CMBS | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2	2.45
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | CMBS | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	9.25	10
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | CMBS | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.04	3.97
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Corporate | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 2,593	$ 2,765
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Corporate | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.05	0.88
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Corporate | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	14.45	8.11
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Corporate | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	3.08	2.89
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-Sale	$ 5	$ 7
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.84	0.88
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Minimum | Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02	0.02
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Minimum | Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.01	0.01
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Minimum | Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.37	0.37
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.01	5.51
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Maximum | Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.05	0.05
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Maximum | Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.05	0.05
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Maximum | Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.80	0.80
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.14	1.72
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Weighted Average [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.04	0.04
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Weighted Average [Member] | Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02	0.02
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | RMBS | Weighted Average [Member] | Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.51	0.50
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Short-term investments | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other Short-term Investments	$ 688	$ 382
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Short-term investments | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	1.54	1.64
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Short-term investments | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	4.88	1.86
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Short-term investments | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	1.85	1.77
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Equity Securities [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Securities, FV-NI	$ 14	$ 14
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Equity Securities [Member] | Valuation, Cost Approach
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Securities, FV-NI	$ 73	$ 56
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Equity Securities [Member] | Minimum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Securities, FV-NI, Measurement Input	14.22	13.70
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Equity Securities [Member] | Maximum | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Securities, FV-NI, Measurement Input	14.22	13.70
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member] | Equity Securities [Member] | Weighted Average [Member] | Measurement Input, Credit Spread [Member] | Valuation Technique, Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Securities, FV-NI, Measurement Input	14.22	13.70