Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned premiums	$ 1,247	$ 1,191	$ 3,696	$ 3,518
Fee revenues	3	5	12	11
Investment income, net of expenses	153	148	453	442
Realized investment gains and losses, net	7	56	156	161
Other	2	2	4	5
Total revenues	1,412	1,402	4,321	4,137
Income (loss) before income taxes	129	253	533	684
Identifiable assets	21,592		21,592		$ 20,386
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,997		2,997		2,967
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,424		1,424		1,366
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,693		16,693		15,569
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	2	2	4	5
Total revenues	34	15	84	38
Income (loss) before income taxes	(46)	(8)	(69)	(38)
Identifiable assets	478		478		$ 484
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	32	13	80	33
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	792	779	2,369	2,310
Fee revenues	1	1	3	3
Total revenues	793	780	2,372	2,313
Income (loss) before income taxes	39	72	61	148
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	314	293	921	864
Fee revenues	1	1	4	3
Total revenues	315	294	925	867
Income (loss) before income taxes	(9)	(8)	(48)	0
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	53	48	153	136
Fee revenues	0	1	1	1
Total revenues	53	49	154	137
Income (loss) before income taxes	13	20	50	42
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	56	58	173	175
Fee revenues	1	2	4	4
Total revenues	57	60	177	179
Income (loss) before income taxes	(4)	(4)	0	(4)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	153	148	453	442
Realized investment gains and losses, net	7	56	156	161
Total revenues	160	204	609	603
Income (loss) before income taxes	136	181	539	536
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	268	265	804	785
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	225	217	674	646
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	159	151	472	442
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	84	90	254	268
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	56	56	165	169
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	148	137	433	403
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	131	122	384	362
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 35	$ 34	$ 104	$ 99