Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information
Assets	$ 24,742		$ 24,742		$ 21,935
Earned premiums	1,446	$ 1,298	4,163	$ 3,852
Fee revenues	4	3	11	11
Other revenues	3	2	7	4
Investment income, net of expenses	161	154	478	458
Investment gains and losses, net	86	458	1,113	372
Total revenues	1,700	1,915	5,772	4,697
Income Before Income Taxes	294	618	1,691	832
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Assets	3,364		3,364		3,285
Life Insurance Segment
Segment Reporting Information
Assets	1,536		1,536		1,424
Investments Segment
Segment Reporting Information
Assets	18,906		18,906		16,741
Corporate, Non-Segment
Segment Reporting Information
Assets	936		936		$ 485
Other revenues	3	2	7	4
Total revenues	119	36	252	97
Income Before Income Taxes	(4)	(15)	(16)	(33)
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	116	34	245	93
Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	834	805	2,467	2,407
Fee revenues	1	1	3	3
Total revenues	835	806	2,470	2,410
Income Before Income Taxes	56	34	144	96
Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	354	338	1,046	994
Fee revenues	1	1	3	4
Total revenues	355	339	1,049	998
Income Before Income Taxes	3	(9)	4	(50)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	72	60	202	173
Fee revenues	1	0	2	1
Total revenues	73	60	204	174
Income Before Income Taxes	12	17	40	48
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	70	61	203	185
Fee revenues	1	1	3	3
Total revenues	71	62	206	188
Income Before Income Taxes	5	3	2	13
Operating Segments | Investments Segment
Segment Reporting Information
Investment income, net of expenses	161	154	478	458
Investment gains and losses, net	86	458	1,113	372
Total revenues	247	612	1,591	830
Income Before Income Taxes	222	588	1,517	758
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	277	268	822	805
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	241	229	709	688
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	179	168	524	495
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	73	80	224	245
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	64	60	188	174
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	156	155	466	459
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	154	142	450	417
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	$ 44	$ 41	$ 130	$ 118