Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 1,260	$ 1,208
Fee revenues	4	5
Investment income, net of expenses	150	149
Investment gains and losses, net	(191)	160
Other	1	1
Total revenues	1,224	1,523
Income (loss) before income taxes	(50)	276
Identifiable assets	21,470		$ 21,843
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,888		2,863
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,418		1,409
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,748		17,112
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Other	1	1
Total revenues	30	23
Income (loss) before income taxes	(11)	(11)
Identifiable assets	416		$ 459
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information [Line Items]
Earned premiums	29	22
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	790	781
Fee revenues	2	1
Total revenues	792	782
Income (loss) before income taxes	15	(2)
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	325	300
Fee revenues	1	2
Total revenues	326	302
Income (loss) before income taxes	(9)	(15)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	56	48
Income (loss) before income taxes	18	18
Operating Segments | Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	60	57
Fee revenues	1	2
Total revenues	61	59
Income (loss) before income taxes	2	0
Operating Segments | Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	150	149
Investment gains and losses, net	(191)	160
Total revenues	(41)	309
Income (loss) before income taxes	(65)	286
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	265	265
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	228	223
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	161	155
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	80	84
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	56	54
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	151	141
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	136	125
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 38	$ 34