Property Casualty Loss and Loss Expense - Claims Development - Commercial Casualty (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 ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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 Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	6,497
Cumulative paid losses and ALAE, net of reinsurance	3,680
All outstanding liabilities before, net of reinsurance	91
Liabilities for loss and ALAE, net of reinsurance	2,908
2014 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	464	$ 462	$ 466	$ 469	$ 465	$ 479	$ 476	$ 479	$ 496	$ 503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 447	437	421	409	390	338	287	172	97	$ 34
2015 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	513	496	504	502	508	516	529	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 14
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 471	453	424	404	362	287	200	108	$ 38
2016 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	525	531	538	554	555	557	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 19
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 485	466	434	395	331	228	126	$ 46
2017 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	553	554	555	571	577	597	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 29
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 486	437	392	320	234	122	$ 48
2018 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	618	612	588	622	641	650
Total of incurred but not reported liabilities plus expected development on reported losses	$ 55
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 505	441	345	253	148	$ 44
2019 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	682	669	607	643	672
Total of incurred but not reported liabilities plus expected development on reported losses	$ 80
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 503	394	259	134	$ 39
2020 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	593	606	629	674
Total of incurred but not reported liabilities plus expected development on reported losses	$ 112
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 345	242	102	$ 33
2021 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	697	697	714
Total of incurred but not reported liabilities plus expected development on reported losses	$ 265
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 251	123	$ 31
2022 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	902	924
Total of incurred but not reported liabilities plus expected development on reported losses	$ 453
Cumulative number of reported claims | reported_claim	13
Cumulative paid losses and ALAE, net of reinsurance	$ 141	$ 37
2023 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	950
Total of incurred but not reported liabilities plus expected development on reported losses	$ 729
Cumulative number of reported claims | reported_claim	8
Cumulative paid losses and ALAE, net of reinsurance	$ 46