Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Earned premiums	$ 1,173	$ 1,111	$ 2,327	$ 2,205
Fee revenues	3	3	6	6
Investment income, net of expenses	149	140	294	279
Realized investment gains, net	44	60	105	107
Other	2	2	3	4
Total revenues	1,371	1,316	2,735	2,601
Income (loss) before income taxes	166	248	431	422
Identifiable assets	20,151		20,151		$ 18,888
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,888		2,888		2,717
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,422		1,422		1,325
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	15,506		15,506		14,485
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	2	2	3	4
Total revenues	12	2	23	4
Income (loss) before income taxes	(17)	(14)	(30)	(29)
Identifiable assets	335		335		$ 361
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	10	0	20	0
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	771	745	1,531	1,478
Fee revenues	1	1	2	2
Total revenues	772	746	1,533	1,480
Income (loss) before income taxes	26	97	76	123
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	288	272	571	540
Fee revenues	1	0	2	1
Total revenues	289	272	573	541
Income (loss) before income taxes	(20)	(25)	8	(28)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	45	42	88	82
Fee revenues	0	1	0	1
Total revenues	45	43	88	83
Income (loss) before income taxes	5	11	22	15
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	59	52	117	105
Fee revenues	1	1	2	2
Total revenues	60	53	119	107
Income (loss) before income taxes	1	1	0	(2)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	149	140	294	279
Realized investment gains, net	44	60	105	107
Total revenues	193	200	399	386
Income (loss) before income taxes	171	178	355	343
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	263	252	520	496
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	215	203	429	399
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	147	139	291	275
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	89	90	178	183
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	57	61	113	125
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	135	125	266	248
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	121	114	240	228
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 32	$ 33	$ 65	$ 64