Investments - Schedule of Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value for Fixed Income Securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	$ 56,732	$ 53,616
Gross unrealized gains	848	361
Gross unrealized losses	(394)	(1,230)
Fair value	57,186	52,747
U.S. government and agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	13,490	11,423
Gross unrealized gains	55	15
Gross unrealized losses	(20)	(330)
Fair value	13,525	11,108
Municipal
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	6,258	8,985
Gross unrealized gains	67	33
Gross unrealized losses	(99)	(176)
Fair value	6,226	8,842
Corporate
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	32,484	30,630
Gross unrealized gains	656	272
Gross unrealized losses	(260)	(710)
Fair value	32,880	30,192
Foreign government
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	1,422	1,352
Gross unrealized gains	23	22
Gross unrealized losses	(10)	(10)
Fair value	1,435	1,364
ABS
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	976	1,130
Gross unrealized gains	13	19
Gross unrealized losses	(3)	(4)
Fair value	986	1,145
MBS
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	2,102	96
Gross unrealized gains	34	0
Gross unrealized losses	(2)	0
Fair value	$ 2,134	$ 96