Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned premiums	$ 1,208	$ 1,154
Fee revenues	5	3
Investment income, net of expenses	149	145
Realized investment gains, net	160	61
Other	1	1
Total revenues	1,523	1,364
Income (loss) before income taxes	276	265
Identifiable assets	20,753		$ 20,386
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,886		2,967
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,359		1,366
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,123		15,569
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	1	1
Total revenues	23	11
Income (loss) before income taxes	(11)	(13)
Identifiable assets	385		$ 484
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	22	10
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	781	760
Fee revenues	1	1
Total revenues	782	761
Income (loss) before income taxes	(2)	50
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	300	283
Fee revenues	2	1
Total revenues	302	284
Income (loss) before income taxes	(15)	28
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	48	43
Income (loss) before income taxes	18	17
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	57	58
Fee revenues	2	1
Total revenues	59	59
Income (loss) before income taxes	0	(1)
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	149	145
Realized investment gains, net	160	61
Total revenues	309	206
Income (loss) before income taxes	286	184
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	265	257
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	223	214
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	155	144
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	84	89
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	54	56
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	141	131
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	125	119
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 34	$ 33