Property Casualty Loss and Loss Expense - Claims Development - Personal Auto (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2023 USD ($) reported_claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,496
Liabilities for loss and ALAE, net of reinsurance	8,535
Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,884
Cumulative paid losses and ALAE, net of reinsurance	1,594
All outstanding liabilities before, net of reinsurance	12
Liabilities for loss and ALAE, net of reinsurance	302
2019 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	381	$ 383	$ 380	$ 383	$ 399
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	102
Cumulative paid losses and ALAE, net of reinsurance	$ 373	363	346	314	$ 250
2020 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	277	277	281	305
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	71
Cumulative paid losses and ALAE, net of reinsurance	$ 264	248	225	$ 186
2021 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	342	343	350
Total of incurred but not reported liabilities plus expected development on reported losses	$ 6
Cumulative number of reported claims | reported_claim	80
Cumulative paid losses and ALAE, net of reinsurance	$ 304	278	$ 219
2022 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	418	427
Total of incurred but not reported liabilities plus expected development on reported losses	$ 15
Cumulative number of reported claims | reported_claim	86
Cumulative paid losses and ALAE, net of reinsurance	$ 349	$ 277
2023 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 72
Cumulative number of reported claims | reported_claim	86
Cumulative paid losses and ALAE, net of reinsurance	$ 304