Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information
Earned premiums	$ 1,669	$ 1,522	$ 4,806	$ 4,460
Fee revenues	4	2	11	8
Other revenues	3	3	8	8
Investment income, net of expenses	179	167	528	498
Investment gains and losses, net	(70)	533	954	(132)
Total revenues	1,785	2,227	6,307	4,842
Income before Income Taxes	184	614	1,824	183
Identifiable Assets	29,907		29,907		$ 27,542
Consolidated Property And Casualty Insurance
Segment Reporting Information
Earned premiums	1,596	1,450	4,585	4,242
Identifiable Assets	4,400		4,400		3,838
Life Insurance Segment
Segment Reporting Information
Identifiable Assets	1,631		1,631		1,661
Investment Segment
Segment Reporting Information
Identifiable Assets	22,940		22,940		21,332
Corporate and Other
Segment Reporting Information
Identifiable Assets	936		936		$ 711
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	930	865	2,727	2,598
Fee revenues	1	1	3	3
Total revenues	931	866	2,730	2,601
Income Before Income Taxes	182	(20)	457	(32)
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	323	290	938	868
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums	264	252	776	755
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	200	189	591	563
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	66	64	201	207
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	77	70	221	205
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	388	367	1,146	1,090
Fee revenues	1	1	3	3
Total revenues	389	368	1,149	1,093
Income Before Income Taxes	(10)	(2)	16	(24)
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	153	154	457	462
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums	184	165	536	487
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	51	48	153	141
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	105	82	289	238
Fee revenues	1	0	2	1
Total revenues	106	82	291	239
Income Before Income Taxes	7	11	25	19
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	73	72	221	218
Fee revenues	1	0	3	1
Total revenues	74	72	224	219
Income Before Income Taxes	(5)	6	(9)	9
Operating Segments | Investment Segment
Segment Reporting Information
Investment income, net of expenses	179	167	528	498
Investment gains and losses, net	(70)	533	954	(132)
Total revenues	109	700	1,482	366
Income Before Income Taxes	83	674	1,403	289
Corporate, Non-Segment
Segment Reporting Information
Total revenues	176	139	431	324
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	173	136	423	316
Corporate, Non-Segment | Corporate and Other
Segment Reporting Information
Other revenues	3	3	8	8
Income Before Income Taxes	$ (73)	$ (55)	$ (68)	$ (78)