Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 1,294	$ 1,241	$ 2,554	$ 2,449
Fee revenues	4	4	8	9
Investment income, net of expenses	154	151	304	300
Investment gains and losses, net	105	(11)	(86)	149
Other	1	1	2	2
Total revenues	1,558	1,386	2,782	2,909
Income (loss) before income taxes	264	128	214	404
Identifiable assets	21,760		21,760		$ 21,843
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	3,011		3,011		2,863
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,406		1,406		1,409
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,972		16,972		17,112
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	1	1	2	2
Total revenues	31	27	61	50
Income (loss) before income taxes	(7)	(12)	(18)	(23)
Identifiable assets	371		371		$ 459
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	30	26	59	48
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	812	796	1,602	1,577
Fee revenues	0	1	2	2
Total revenues	812	797	1,604	1,579
Income (loss) before income taxes	47	24	62	22
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	331	307	656	607
Fee revenues	2	1	3	3
Total revenues	333	308	659	610
Income (loss) before income taxes	(32)	(24)	(41)	(39)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	57	52	113	100
Fee revenues	1	1	1	1
Total revenues	58	53	114	101
Income (loss) before income taxes	13	19	31	37
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	64	60	124	117
Fee revenues	1	1	2	3
Total revenues	65	61	126	120
Income (loss) before income taxes	8	4	10	4
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	154	151	304	300
Investment gains and losses, net	105	(11)	(86)	149
Total revenues	259	140	218	449
Income (loss) before income taxes	235	117	170	403
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	272	271	537	536
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	231	226	459	449
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	166	158	327	313
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	85	86	165	170
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	58	55	114	109
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	153	144	304	285
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	139	128	275	253
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 39	$ 35	$ 77	$ 69