Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earned premiums	$ 1,127	$ 1,071	$ 3,332	$ 3,157
Fee revenues	4	3	10	9
Investment income, net of expenses	143	138	422	409
Realized investment gains, net	3	65	110	101
Other	1	3	5	7
Total revenues	1,278	1,280	3,879	3,683
Income (loss) before income taxes	243	259	665	485
Identifiable assets	18,751		18,751		$ 18,753
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,568		2,568		2,656
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,328		1,328		1,316
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	14,476		14,476		14,441
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	757	720	2,235	2,126
Fee revenues	1	1	3	3
Total revenues	758	721	2,238	2,129
Income (loss) before income taxes	121	67	244	97
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	277	263	817	775
Fee revenues	1	1	2	2
Total revenues	278	264	819	777
Income (loss) before income taxes	(2)	14	(30)	(33)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	42	42	124	109
Fee revenues	0	0	1	0
Total revenues	42	42	125	109
Income (loss) before income taxes	14	13	29	22
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	51	46	156	147
Fee revenues	2	1	4	4
Total revenues	53	47	160	151
Income (loss) before income taxes	1	(4)	(1)	(5)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	143	138	422	409
Realized investment gains, net	3	65	110	101
Total revenues	146	203	532	510
Income (loss) before income taxes	125	182	468	448
Corporate and Other Non-Segment
Segment Reporting Information [Line Items]
Other	1	3	5	7
Income (loss) before income taxes	(16)	(13)	(45)	(44)
Identifiable assets	379		379		$ 340
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	257	237	753	695
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	205	186	604	537
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	141	133	416	391
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	93	93	276	280
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	61	71	186	223
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	128	121	376	354
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	117	110	345	330
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 32	$ 32	$ 96	$ 91