Segment Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) segment industry	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting Information
Earned premiums	$ 7,958	$ 7,225	$ 6,478
Fee revenues	21	14	15
Other	13	10	10
Investment income, net of expenses	894	781	714
Total revenues	10,013	6,563	9,626
Total income (loss) before income taxes	2,276	(694)	3,698
Total assets	$ 32,769	29,732
Number of Industries Operated In | industry	2
Number of Insurance Segments | segment	4
Life Insurance Segment
Segment Reporting Information
Earned premiums	$ 313	301	294
Operating Segments
Segment Reporting Information
Total assets	5,294	5,178
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	4,264	4,024	3,674
Fee revenues	4	4	4
Total revenues	4,268	4,028	3,678
Income (loss) before income taxes	168	38	598
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	1,481	1,416	1,270
Operating Segments | Commercial Insurance | Commercial Property Segment
Segment Reporting Information
Earned premiums	1,264	1,136	1,043
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	862	842	794
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	277	284	268
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	380	346	299
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	2,044	1,689	1,542
Fee revenues	4	4	4
Total revenues	2,048	1,693	1,546
Income (loss) before income taxes	(4)	18	97
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	721	626	609
Operating Segments | Personal Insurance | Homeowner
Segment Reporting Information
Earned premiums	1,044	829	726
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	279	234	207
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	542	485	398
Fee revenues	3	2	2
Total revenues	545	487	400
Income (loss) before income taxes	54	48	44
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	313	301	294
Fee revenues	10	4	5
Total revenues	323	305	299
Income (loss) before income taxes	41	27	12
Total assets	1,562	1,518
Operating Segments | Investments Segment
Segment Reporting Information
Investment income, net of expenses	894	781	714
Investment gains and losses, net	1,127	(1,467)	2,409
Total revenues	2,021	(686)	3,123
Income (loss) before income taxes	1,900	(795)	3,018
Total assets	24,999	22,133
Corporate, Non-Segment
Segment Reporting Information
Earned premiums	795	726	570
Other	13	10	10
Total revenues	808	736	580
Income (loss) before income taxes	117	(30)	$ (71)
Total assets	$ 914	$ 903