Property Casualty Loss and Loss Expense - Claims Development - Commercial 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
Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,432
Cumulative paid losses and ALAE, net of reinsurance	1,657
All outstanding liabilities before, net of reinsurance	32
Liabilities for loss and ALAE, net of reinsurance	807
2019 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	471	$ 469	$ 453	$ 451	$ 452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 7
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 435	395	333	268	$ 183
2020 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	377	384	391	424
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	36
Cumulative paid losses and ALAE, net of reinsurance	$ 328	280	214	$ 154
2021 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	465	477	470
Total of incurred but not reported liabilities plus expected development on reported losses	$ 51
Cumulative number of reported claims | reported_claim	39
Cumulative paid losses and ALAE, net of reinsurance	$ 346	278	$ 179
2022 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	569	558
Total of incurred but not reported liabilities plus expected development on reported losses	$ 105
Cumulative number of reported claims | reported_claim	41
Cumulative paid losses and ALAE, net of reinsurance	$ 332	$ 217
2023 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	550
Total of incurred but not reported liabilities plus expected development on reported losses	$ 203
Cumulative number of reported claims | reported_claim	34
Cumulative paid losses and ALAE, net of reinsurance	$ 216