Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information
Earned premiums	$ 1,593	$ 1,482	$ 3,137	$ 2,938
Fee revenues	4	3	7	6
Other revenues	3	3	5	5
Investment income, net of expenses	175	166	349	331
Investment gains and losses, net	520	1,060	1,024	(665)
Total revenues	2,295	2,714	4,522	2,615
Income Before Income Taxes	872	1,145	1,640	(431)
Identifiable Assets	29,677		29,677		$ 27,542
Consolidated Property And Casualty Insurance
Segment Reporting Information
Earned premiums	1,514	1,403	2,989	2,792
Identifiable Assets	4,376		4,376		3,838
Life Insurance Segment
Segment Reporting Information
Identifiable Assets	1,580		1,580		1,661
Investment Segment
Segment Reporting Information
Identifiable Assets	22,825		22,825		21,332
Corporate and Other
Segment Reporting Information
Identifiable Assets	896		896		$ 711
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	911	870	1,797	1,733
Fee revenues	1	1	2	2
Total revenues	912	871	1,799	1,735
Income Before Income Taxes	145	8	275	(12)
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	312	289	615	577
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums	259	254	512	504
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	198	189	391	374
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	68	68	135	143
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	74	70	144	135
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	382	364	758	723
Fee revenues	1	1	2	2
Total revenues	383	365	760	725
Income Before Income Taxes	29	(43)	26	(22)
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	152	154	305	308
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums	178	163	352	322
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	52	47	101	93
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	95	78	184	156
Fee revenues	1	0	1	1
Total revenues	96	78	185	157
Income Before Income Taxes	10	(1)	18	8
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	79	79	148	146
Fee revenues	1	1	2	1
Total revenues	80	80	150	147
Income Before Income Taxes	(2)	1	(4)	3
Operating Segments | Investment Segment
Segment Reporting Information
Investment income, net of expenses	175	166	349	331
Investment gains and losses, net	520	1,060	1,024	(665)
Total revenues	695	1,226	1,373	(334)
Income Before Income Taxes	668	1,201	1,320	(385)
Corporate, Non-Segment
Segment Reporting Information
Total revenues	129	94	255	185
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	126	91	250	180
Corporate, Non-Segment | Corporate and Other
Segment Reporting Information
Other revenues	3	3	5	5
Income Before Income Taxes	$ 22	$ (21)	$ 5	$ (23)