Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information
Assets	$ 24,337		$ 24,337		$ 21,935
Earned premiums	1,384	$ 1,294	2,717	$ 2,554
Fee revenues	3	4	7	8
Other revenues	2	1	4	2
Investment income, net of expenses	160	154	317	304
Investment gains and losses, net	364	105	1,027	(86)
Total revenues	1,913	1,558	4,072	2,782
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	530	264	1,397	214
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Assets	3,441		3,441		3,285
Life Insurance Segment
Segment Reporting Information
Assets	1,536		1,536		1,424
Investments Segment
Segment Reporting Information
Assets	18,464		18,464		16,741
Corporate, Non-Segment
Segment Reporting Information
Assets	896		896		$ 485
Other revenues	2	1	4	2
Total revenues	81	31	133	61
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(1)	(7)	(12)	(18)
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	79	30	129	59
Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	823	812	1,633	1,602
Fee revenues	1	0	2	2
Total revenues	824	812	1,635	1,604
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	12	47	88	62
Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	348	331	692	656
Fee revenues	1	2	2	3
Total revenues	349	333	694	659
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	5	(32)	1	(41)
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	67	57	130	113
Fee revenues	0	1	1	1
Total revenues	67	58	131	114
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	17	13	28	31
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	67	64	133	124
Fee revenues	1	1	2	2
Total revenues	68	65	135	126
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(2)	8	(3)	10
Operating Segments | Investments Segment
Segment Reporting Information
Investment income, net of expenses	160	154	317	304
Investment gains and losses, net	364	105	1,027	(86)
Total revenues	524	259	1,344	218
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	499	235	1,295	170
Commercial Casualty | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	277	272	545	537
Commercial Property | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	234	231	468	459
Commercial Auto | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	175	166	345	327
Workers' Compensation | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	74	85	151	165
Other Commercial Lines | Operating Segments | Commercial Lines
Segment Reporting Information
Earned premiums	63	58	124	114
Personal Auto | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	155	153	310	304
Home Owner Segment | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	149	139	296	275
Other Personal Lines | Operating Segments | Personal Lines
Segment Reporting Information
Earned premiums	$ 44	$ 39	$ 86	$ 77