Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Segment Reporting Information
Earned premiums	$ 2,071	$ 1,918
Fee revenues	4	4
Other	3	3
Investment income, net of expenses	245	210
Investment gains and losses, net	612	106
Total revenues	2,935	2,241
Income before Income Taxes	953	268
Assets	33,727		$ 32,769
Operating Segments
Segment Reporting Information
Assets	5,298		5,294
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	1,082	1,056
Fee revenues	1	1
Total revenues	1,083	1,057
Income Before Income Taxes	39	(2)
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	365	377
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums	336	299
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	220	213
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	61	74
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	100	93
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	588	464
Fee revenues	1	1
Total revenues	589	465
Income Before Income Taxes	37	(57)
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	208	166
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums	303	232
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	77	66
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	139	127
Fee revenues	1	0
Total revenues	140	127
Income Before Income Taxes	12	13
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	79	77
Fee revenues	1	2
Total revenues	80	79
Income Before Income Taxes	10	8
Assets	1,564		1,562
Operating Segments | Investment Segment
Segment Reporting Information
Investment income, net of expenses	245	210
Investment gains and losses, net	612	106
Total revenues	857	316
Income Before Income Taxes	826	286
Assets	25,878		24,999
Corporate, Non-Segment
Segment Reporting Information
Earned premiums	183	194
Other	3	3
Total revenues	186	197
Income Before Income Taxes	29	$ 20
Assets	$ 987		$ 914