Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment industry	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information
Earned premiums									$ 5,980	$ 5,604	$ 5,170
Fee revenues									11	15	15
Other Income									10	9	5
Total revenues	$ 2,694	$ 2,227	$ 2,714	$ (99)	$ 2,152	$ 1,700	$ 1,913	$ 2,159	7,536	7,924	5,407
Net Investment Income									670	646	619
Investment gains and losses, net									865	1,650	(402)
Income (loss) before income taxes	1,316	$ 614	$ 1,145	$ (1,576)	781	$ 294	$ 530	$ 867	1,499	2,472	251
Identifiable assets	27,542				25,408				$ 27,542	25,408
Number of Industries Operated In | industry									2
Number of Insurance Segments | segment									4
Consolidated Property And Casualty Insurance Entity
Segment Reporting Information
Identifiable assets	3,838				3,437				$ 3,838	3,437
Life Insurance Segment
Segment Reporting Information
Earned premiums									289	270	250
Identifiable assets	1,661				1,516				1,661	1,516
Investments Segment
Segment Reporting Information
Identifiable assets	21,332				19,583				21,332	19,583
Corporate and Other [Member]
Segment Reporting Information
Identifiable assets	$ 711				$ 872				711	872
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums									3,476	3,319	3,218
Fee revenues									3	5	5
Total revenues									3,479	3,324	3,223
Income (loss) before income taxes									64	241	151
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums									1,165	1,102	1,075
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums									1,010	958	920
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums									755	707	664
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums									271	300	324
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums									275	252	235
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums									1,463	1,404	1,336
Fee revenues									4	4	5
Total revenues									1,467	1,408	1,341
Income (loss) before income taxes									47	8	(20)
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums									615	621	614
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums									658	607	563
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums									190	176	159
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums									325	278	234
Fee revenues									2	2	1
Total revenues									327	280	235
Income (loss) before income taxes									34	53	63
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums									289	270	250
Fee revenues									2	4	4
Total revenues									291	274	254
Income (loss) before income taxes									11	1	8
Operating Segments | Investments Segment
Segment Reporting Information
Total revenues									1,535	2,296	217
Net Investment Income									670	646	619
Investment gains and losses, net									865	1,650	(402)
Income (loss) before income taxes									1,433	2,197	121
Corporate, Non-Segment
Segment Reporting Information
Total revenues									437	342	137
Corporate, Non-Segment | Reinsurance assumed and other non segment
Segment Reporting Information
Earned premiums									427	333	132
Corporate, Non-Segment | Corporate and Other [Member]
Segment Reporting Information
Other Income									10	9	5
Income (loss) before income taxes									$ (90)	$ (28)	$ (72)