Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 1,915	$ 1,412	$ 4,697	$ 4,321
Income (loss) before income taxes	618	129	832	533
Identifiable assets	22,480		22,480		$ 21,843
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	3,129		3,129		2,863
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,418		1,418		1,409
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	17,457		17,457		17,112
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	36	34	97	84
Income (loss) before income taxes	(15)	(46)	(33)	(69)
Identifiable assets	476		476		$ 459
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	806	793	2,410	2,372
Income (loss) before income taxes	34	39	96	61
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	339	315	998	925
Income (loss) before income taxes	(9)	(9)	(50)	(48)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Total revenues	60	53	174	154
Income (loss) before income taxes	17	13	48	50
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Total revenues	62	57	188	177
Income (loss) before income taxes	3	(4)	13	0
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Total revenues	612	160	830	609
Income (loss) before income taxes	588	136	758	539
Insurance premiums
Segment Reporting Information [Line Items]
Total revenues	1,298	1,247	3,852	3,696
Insurance premiums | Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Total revenues	34	32	93	80
Insurance premiums | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	805	792	2,407	2,369
Insurance premiums | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	338	314	994	921
Insurance premiums | Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Total revenues	60	53	173	153
Insurance premiums | Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Total revenues	61	56	185	173
Insurance premiums | Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	268	268	805	804
Insurance premiums | Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	229	225	688	674
Insurance premiums | Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	168	159	495	472
Insurance premiums | Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	80	84	245	254
Insurance premiums | Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	60	56	174	165
Insurance premiums | Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	155	148	459	433
Insurance premiums | Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	142	131	417	384
Insurance premiums | Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	41	35	118	104
Fee revenues
Segment Reporting Information [Line Items]
Total revenues	3	3	11	12
Fee revenues | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	1	1	3	3
Fee revenues | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total revenues	1	1	4	4
Fee revenues | Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Total revenues	0	0	1	1
Fee revenues | Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Total revenues	1	1	3	4
Investment income, net of expenses
Segment Reporting Information [Line Items]
Total revenues	154	153	458	453
Investment income, net of expenses | Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Total revenues	154	153	458	453
Investment gains and losses, net
Segment Reporting Information [Line Items]
Total revenues	458	7	372	156
Investment gains and losses, net | Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Total revenues	458	7	372	156
Other
Segment Reporting Information [Line Items]
Total revenues	2	2	4	4
Other | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 2	$ 2	$ 4	$ 4