Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Segment Reporting Information
Earned premiums	$ 2,033	$ 1,884	$ 5,894	$ 5,350
Fee revenues	6	5	16	12
Investment income, net of expenses	225	193	655	573
Investment gains and losses, net	(456)	(674)	84	(2,494)
Other revenues	3	2	8	7
Total revenues	1,811	1,410	6,657	3,448
Income before Income Taxes	(148)	(577)	786	(1,981)
Assets	30,915		30,915		$ 29,732
Operating Segments
Segment Reporting Information
Assets	5,302		5,302		5,178
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	1,062	1,028	3,184	2,984
Fee revenues	1	1	3	3
Total revenues	1,063	1,029	3,187	2,987
Income Before Income Taxes	52	11	83	25
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	365	360	1,115	1,046
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums	321	292	933	846
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	216	213	644	627
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	66	73	212	209
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	94	90	280	256
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	527	431	1,484	1,246
Fee revenues	1	1	3	3
Total revenues	528	432	1,487	1,249
Income Before Income Taxes	1	(18)	(92)	(2)
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	185	158	524	465
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums	271	213	755	609
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	71	60	205	172
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	135	125	394	361
Fee revenues	1	1	2	2
Total revenues	136	126	396	363
Income Before Income Taxes	14	9	38	44
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	76	75	233	226
Fee revenues	3	2	8	4
Total revenues	79	77	241	230
Income Before Income Taxes	17	13	38	21
Assets	1,556		1,556		1,518
Operating Segments | Investment Segment
Segment Reporting Information
Investment income, net of expenses	225	193	655	573
Investment gains and losses, net	(456)	(674)	84	(2,494)
Total revenues	(231)	(481)	739	(1,921)
Income Before Income Taxes	(262)	(508)	648	(2,003)
Assets	23,063		23,063		22,133
Corporate, Non-Segment
Segment Reporting Information
Earned premiums	233	225	599	533
Other revenues	3	2	8	7
Total revenues	236	227	607	540
Income Before Income Taxes	30	$ (84)	71	$ (66)
Assets	$ 994		$ 994		$ 903