Income Taxes - Schedule of Components of the Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets
Unearned premium reserves	$ 897	$ 815
Discount on loss reserves	269	228
Net operating loss carryover	258	97
Research & development capitalization	228	219
Unrealized net capital losses	174	609
Accrued compensation	121	128
General business credits carryover	110	0
Pension	26	10
Other postretirement benefits	13	15
Other	110	95
Total deferred tax assets before valuation allowance	2,206	2,216
Valuation allowance	(69)	(34)
Total deferred tax assets after valuation allowance	2,137	2,182
Deferred tax liabilities
DAC	(1,075)	(1,018)
Investments	(455)	(431)
Intangible assets	(99)	(147)
Other	(289)	(204)
Total deferred tax liabilities	(1,918)	(1,800)
Net deferred tax assets	$ 219	$ 382