Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information
Assets	$ 25,450		$ 25,450		$ 25,408
Earned premiums	1,482	$ 1,384	2,938	$ 2,717
Fee revenues	3	3	6	7
Other revenues	3	2	5	4
Investment income, net of expenses	166	160	331	317
Investment gains and losses, net	1,060	364	(665)	1,027
Total revenues	2,714	1,913	2,615	4,072
Income Before Income Taxes	1,145	530	(431)	1,397
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Assets	3,751		3,751		3,437
Earned premiums	1,403	1,317	2,792	2,584
Life Insurance Segment
Segment Reporting Information
Assets	1,570		1,570		1,516
Investments Segment
Segment Reporting Information
Assets	19,375		19,375		19,583
Corporate, Non-Segment
Segment Reporting Information
Assets	754		754		$ 872
Other revenues	3	2	5	4
Total revenues	94	81	185	133
Income Before Income Taxes	(21)	(1)	(23)	(12)
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	91	79	180	129
Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	870	823	1,733	1,633
Fee revenues	1	1	2	2
Total revenues	871	824	1,735	1,635
Income Before Income Taxes	8	12	(12)	88
Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	364	348	723	692
Fee revenues	1	1	2	2
Total revenues	365	349	725	694
Income Before Income Taxes	(43)	5	(22)	1
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	78	67	156	130
Fee revenues	0	0	1	1
Total revenues	78	67	157	131
Income Before Income Taxes	(1)	17	8	28
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	79	67	146	133
Fee revenues	1	1	1	2
Total revenues	80	68	147	135
Income Before Income Taxes	1	(2)	3	(3)
Operating Segments | Investments Segment
Segment Reporting Information
Investment income, net of expenses	166	160	331	317
Investment gains and losses, net	1,060	364	(665)	1,027
Total revenues	1,226	524	(334)	1,344
Income Before Income Taxes	1,201	499	(385)	1,295
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	289	277	577	545
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	254	234	504	468
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	189	175	374	345
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	68	74	143	151
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	70	63	135	124
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	154	155	308	310
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	163	149	322	296
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	$ 47	$ 44	$ 93	$ 86