Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Earned premiums	$ 1,191	$ 1,127	$ 3,518	$ 3,332
Fee revenues	5	4	11	10
Investment income, net of expenses	148	143	442	422
Realized investment gains, net	56	3	161	110
Other	2	1	5	5
Total revenues	1,402	1,278	4,137	3,879
Income (loss) before income taxes	253	243	684	665
Identifiable assets	20,455		20,455		$ 18,888
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,857		2,857		2,717
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,417		1,417		1,325
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	15,682		15,682		14,485
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	2	1	5	5
Total revenues	15	1	38	5
Income (loss) before income taxes	(8)	(16)	(38)	(45)
Identifiable assets	499		499		$ 361
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	13	0	33	0
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	779	757	2,310	2,235
Fee revenues	1	1	3	3
Total revenues	780	758	2,313	2,238
Income (loss) before income taxes	72	121	148	244
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	293	277	864	817
Fee revenues	1	1	3	2
Total revenues	294	278	867	819
Income (loss) before income taxes	(8)	(2)	0	(30)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	48	42	136	124
Fee revenues	1	0	1	1
Total revenues	49	42	137	125
Income (loss) before income taxes	20	14	42	29
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	58	51	175	156
Fee revenues	2	2	4	4
Total revenues	60	53	179	160
Income (loss) before income taxes	(4)	1	(4)	(1)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	148	143	442	422
Realized investment gains, net	56	3	161	110
Total revenues	204	146	603	532
Income (loss) before income taxes	181	125	536	468
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	265	257	785	753
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	217	205	646	604
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	151	141	442	416
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	90	93	268	276
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	56	61	169	186
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	137	128	403	376
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	122	117	362	345
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 34	$ 32	$ 99	$ 96