Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned premiums	$ 1,241	$ 1,173	$ 2,449	$ 2,327
Fee revenues	4	3	9	6
Investment income, net of expenses	151	149	300	294
Realized investment gains and losses, net	(11)	44	149	105
Other	1	2	2	3
Total revenues	1,386	1,371	2,909	2,735
Income (loss) before income taxes	128	166	404	431
Identifiable assets	21,238		21,238		$ 20,386
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	3,042		3,042		2,967
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,427		1,427		1,366
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,431		16,431		15,569
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	1	2	2	3
Total revenues	27	12	50	23
Income (loss) before income taxes	(12)	(17)	(23)	(30)
Identifiable assets	338		338		$ 484
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	26	10	48	20
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	796	771	1,577	1,531
Fee revenues	1	1	2	2
Total revenues	797	772	1,579	1,533
Income (loss) before income taxes	24	26	22	76
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	307	288	607	571
Fee revenues	1	1	3	2
Total revenues	308	289	610	573
Income (loss) before income taxes	(24)	(20)	(39)	8
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	52	45	100	88
Fee revenues	1	0	1	0
Total revenues	53	45	101	88
Income (loss) before income taxes	19	5	37	22
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	60	59	117	117
Fee revenues	1	1	3	2
Total revenues	61	60	120	119
Income (loss) before income taxes	4	1	4	0
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	151	149	300	294
Realized investment gains and losses, net	(11)	44	149	105
Total revenues	140	193	449	399
Income (loss) before income taxes	117	171	403	355
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	271	263	536	520
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	226	215	449	429
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	158	147	313	291
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	86	89	170	178
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	55	57	109	113
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	144	135	285	266
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	128	121	253	240
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 35	$ 32	$ 69	$ 65