Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earned premiums	$ 1,111	$ 1,059	$ 2,205	$ 2,086
Fee revenues	3	3	6	6
Investment income, net of expenses	140	136	279	271
Realized investment gains, net	60	14	107	36
Other	2	2	4	4
Total revenues	1,316	1,214	2,601	2,403
Income (loss) before income taxes	248	107	422	226
Identifiable assets	18,888		18,888		$ 18,753
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,690		2,690		2,656
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,277		1,277		1,316
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	14,606		14,606		14,441
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	745	714	1,478	1,406
Fee revenues	1	1	2	2
Total revenues	746	715	1,480	1,408
Income (loss) before income taxes	97	28	123	30
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	272	258	540	512
Fee revenues	0	1	1	1
Total revenues	272	259	541	513
Income (loss) before income taxes	(25)	(40)	(28)	(47)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	42	34	82	67
Fee revenues	1	0	1	0
Total revenues	43	34	83	67
Income (loss) before income taxes	11	5	15	9
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	52	53	105	101
Fee revenues	1	1	2	3
Total revenues	53	54	107	104
Income (loss) before income taxes	1	(1)	(2)	(1)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	140	136	279	271
Realized investment gains, net	60	14	107	36
Total revenues	200	150	386	307
Income (loss) before income taxes	178	130	343	266
Corporate and Other Non-Segment
Segment Reporting Information [Line Items]
Other	2	2	4	4
Income (loss) before income taxes	(14)	(15)	(29)	(31)
Identifiable assets	315		315		$ 340
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	252	234	496	458
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	203	180	399	351
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	139	132	275	258
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	90	95	183	187
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	61	73	125	152
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	125	117	248	233
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	114	111	228	220
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 33	$ 30	$ 64	$ 59