Property Casualty Loss and Loss Expense - Claims Development - Commercial Property (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 Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	3,496
Cumulative paid losses and ALAE, net of reinsurance	3,033
All outstanding liabilities before, net of reinsurance	12
Liabilities for loss and ALAE, net of reinsurance	475
2019 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	592	$ 598	$ 600	$ 606	$ 621
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 591	590	579	561	$ 413
2020 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	716	719	742	855
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 687	672	637	$ 489
2021 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	576	586	607
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 558	527	$ 326
2022 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	779	813
Total of incurred but not reported liabilities plus expected development on reported losses	$ 24
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 691	$ 393
2023 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	833
Total of incurred but not reported liabilities plus expected development on reported losses	$ 92
Cumulative number of reported claims | reported_claim	12
Cumulative paid losses and ALAE, net of reinsurance	$ 506