Investments - Schedule of Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value for Fixed Income Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	$ 51,837	$ 49,649
Gross unrealized gains	408	650
Gross unrealized losses	(1,468)	(1,434)
Fair value	50,777	48,865
U.S. government and agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	10,142	8,624
Gross unrealized gains	56	114
Gross unrealized losses	(168)	(119)
Fair value	10,030	8,619
Municipal
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	5,491	6,049
Gross unrealized gains	61	109
Gross unrealized losses	(160)	(152)
Fair value	5,392	6,006
Corporate
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	33,110	31,951
Gross unrealized gains	273	397
Gross unrealized losses	(1,114)	(1,143)
Fair value	32,269	31,205
Foreign government
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	1,255	1,286
Gross unrealized gains	4	17
Gross unrealized losses	(19)	(13)
Fair value	1,240	1,290
ABS
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, net	1,839	1,739
Gross unrealized gains	14	13
Gross unrealized losses	(7)	(7)
Fair value	$ 1,846	$ 1,745