Claims Development - Commercial Casualty (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2021 USD ($) reported_claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,118
Liabilities for loss and ALAE, net of reinsurance	6,917
Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	5,383
Cumulative paid losses and ALAE, net of reinsurance	3,146
All outstanding liabilities before, net of reinsurance	90
Liabilities for loss and ALAE, net of reinsurance	2,327
2012 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	397	$ 397	$ 397	$ 399	$ 404	$ 394	$ 394	$ 417	$ 414	$ 466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 383	374	364	346	330	288	232	170	88	$ 27
2013 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	385	386	391	407	413	416	431	443	448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	466	469	465	479	476	479	496	503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	504	502	508	516	529	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 42
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	538	554	555	557	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 58
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	555	571	577	597	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 68
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	588	622	641	650
Total of incurred but not reported liabilities plus expected development on reported losses	$ 101
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	607	643	672
Total of incurred but not reported liabilities plus expected development on reported losses	$ 141
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 259	134	$ 39
2020 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	629	674
Total of incurred but not reported liabilities plus expected development on reported losses	$ 307
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 102	$ 33
2021 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	714
Total of incurred but not reported liabilities plus expected development on reported losses	$ 492
Cumulative number of reported claims | reported_claim	10
Cumulative paid losses and ALAE, net of reinsurance	$ 31