Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Earned premiums	$ 1,154	$ 1,094
Fee revenues	3	3
Investment income, net of expenses	145	139
Realized investment gains, net	61	47
Other	1	2
Total revenues	1,364	1,285
Income (loss) before income taxes	265	174
Identifiable assets	19,447		$ 18,888
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,699		2,717
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,372		1,325
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	14,978		14,485
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	760	733
Fee revenues	1	1
Total revenues	761	734
Income (loss) before income taxes	50	26
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	283	268
Fee revenues	1	1
Total revenues	284	269
Income (loss) before income taxes	28	(3)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	43	40
Income (loss) before income taxes	17	4
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	58	53
Fee revenues	1	1
Total revenues	59	54
Income (loss) before income taxes	(1)	(3)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	145	139
Realized investment gains, net	61	47
Total revenues	206	186
Income (loss) before income taxes	184	165
Corporate and Other Non-Segment
Segment Reporting Information [Line Items]
Other	11	2
Income (loss) before income taxes	(13)	(15)
Identifiable assets	398		$ 361
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	257	244
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	214	196
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	144	136
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	89	93
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	56	64
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	131	123
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	119	114
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 33	$ 31