Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information
Earned premiums	$ 1,544	$ 1,456
Fee revenues	3	3
Other revenues	2	2
Investment income, net of expenses	174	165
Investment gains and losses, net	504	(1,725)
Total revenues	2,227	(99)
Income Before Income Taxes	768	(1,576)
Identifiable Assets	28,313		$ 27,542
Consolidated Property And Casualty Insurance
Segment Reporting Information
Earned premiums	1,475	1,389
Identifiable Assets	4,163		3,838
Life Insurance Segment
Segment Reporting Information
Identifiable Assets	1,574		1,661
Investment Segment
Segment Reporting Information
Identifiable Assets	21,803		21,332
Corporate and Other
Segment Reporting Information
Identifiable Assets	773		$ 711
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums	886	863
Fee revenues	1	1
Total revenues	887	864
Income Before Income Taxes	130	(20)
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums	303	289
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums	253	249
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums	193	185
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums	67	75
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums	70	65
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums	376	359
Fee revenues	1	1
Total revenues	377	360
Income Before Income Taxes	(3)	21
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums	152	154
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums	174	159
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums	50	46
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums	89	78
Fee revenues	0	1
Total revenues	89	79
Income Before Income Taxes	8	9
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums	69	67
Fee revenues	1	0
Total revenues	70	67
Income Before Income Taxes	(2)	2
Operating Segments | Investment Segment
Segment Reporting Information
Investment income, net of expenses	174	165
Investment gains and losses, net	504	(1,725)
Total revenues	678	(1,560)
Income Before Income Taxes	652	(1,586)
Corporate, Non-Segment
Segment Reporting Information
Total revenues	126	91
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums	124	89
Corporate, Non-Segment | Corporate and Other
Segment Reporting Information
Other revenues	2	2
Income Before Income Taxes	$ (17)	$ (2)