Income Taxes - Schedule of Components of the Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets
Unearned premium reserves	$ 978	$ 897
Research & development capitalization	317	228
Discount on loss reserves	292	269
Net operating loss carryover	232	258
Unrealized net capital losses	209	174
Accrued compensation	131	121
Pension	8	26
Other postretirement benefits	7	13
General business credits carryover	0	110
Other	185	110
Total deferred tax assets before valuation allowance	2,359	2,206
Valuation allowance	(69)	(69)
Total deferred tax assets after valuation allowance	2,290	2,137
Deferred tax liabilities
DAC	(1,173)	(1,075)
Investments	(479)	(455)
Depreciable assets	(123)	(77)
Intangible assets	(66)	(99)
Other	(218)	(212)
Total deferred tax liabilities	(2,059)	(1,918)
Net deferred tax assets	$ 231	$ 219