Claims Development - Commercial Casualty (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2022 USD ($) reported_claim	Dec. 31, 2021 USD ($)	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 ($)	Dec. 31, 2013 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,312
Liabilities for loss and ALAE, net of reinsurance	7,967
Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	5,932
Cumulative paid losses and ALAE, net of reinsurance	3,396
All outstanding liabilities before, net of reinsurance	86
Liabilities for loss and ALAE, net of reinsurance	2,622
2013 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	381	$ 385	$ 386	$ 391	$ 407	$ 413	$ 416	$ 431	$ 443	$ 448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 9
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 366	355	348	337	312	286	232	159	90	$ 35
2014 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	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	$ 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,	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	$ 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,	531	538	554	555	557	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 32
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	554	555	571	577	597	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 51
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	612	588	622	641	650
Total of incurred but not reported liabilities plus expected development on reported losses	$ 77
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	669	607	643	672
Total of incurred but not reported liabilities plus expected development on reported losses	$ 124
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	606	629	674
Total of incurred but not reported liabilities plus expected development on reported losses	$ 188
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 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	714
Total of incurred but not reported liabilities plus expected development on reported losses	$ 338
Cumulative number of reported claims | reported_claim	13
Cumulative paid losses and ALAE, net of reinsurance	$ 123	$ 31
2022 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	924
Total of incurred but not reported liabilities plus expected development on reported losses	$ 613
Cumulative number of reported claims | reported_claim	10
Cumulative paid losses and ALAE, net of reinsurance	$ 37