Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information
Assets	$ 26,370		$ 26,370		$ 25,408
Earned premiums	1,522	$ 1,446	4,460	$ 4,163
Fee revenues	2	4	8	11
Other revenues	3	3	8	7
Investment income, net of expenses	167	161	498	478
Investment gains and losses, net	533	86	(132)	1,113
Total revenues	2,227	1,700	4,842	5,772
Income Before Income Taxes	614	294	183	1,691
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Assets	3,796		3,796		3,437
Earned premiums	1,450	1,376	4,242	3,960
Life Insurance Segment
Segment Reporting Information
Assets	1,619		1,619		1,516
Investments Segment
Segment Reporting Information
Assets	20,155		20,155		19,583
Corporate, Non-Segment
Segment Reporting Information
Assets	800		800		$ 872
Other revenues	3	3	8	7
Total revenues	139	119	324	252
Income Before Income Taxes	(55)	(4)	(78)	(16)
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	136	116	316	245
Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	865	834	2,598	2,467
Fee revenues	1	1	3	3
Total revenues	866	835	2,601	2,470
Income Before Income Taxes	(20)	56	(32)	144
Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	367	354	1,090	1,046
Fee revenues	1	1	3	3
Total revenues	368	355	1,093	1,049
Income Before Income Taxes	(2)	3	(24)	4
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	82	72	238	202
Fee revenues	0	1	1	2
Total revenues	82	73	239	204
Income Before Income Taxes	11	12	19	40
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	72	70	218	203
Fee revenues	0	1	1	3
Total revenues	72	71	219	206
Income Before Income Taxes	6	5	9	2
Operating Segments | Investments Segment
Segment Reporting Information
Investment income, net of expenses	167	161	498	478
Investment gains and losses, net	533	86	(132)	1,113
Total revenues	700	247	366	1,591
Income Before Income Taxes	674	222	289	1,517
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	290	277	868	822
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	252	241	755	709
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	189	179	563	524
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	64	73	207	224
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	70	64	205	188
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	154	156	462	466
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	165	154	487	450
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	$ 48	$ 44	$ 141	$ 130