Claims Development - Commercial Casualty (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2020 USD ($) reported_claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,158
Liabilities for loss and ALAE, net of reinsurance	6,394
Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	5,179
Cumulative paid losses and ALAE, net of reinsurance	3,012
All outstanding liabilities before, net of reinsurance	91
Liabilities for loss and ALAE, net of reinsurance	2,258
2011 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	361	$ 368	$ 365	$ 366	$ 380	$ 375	$ 377	$ 377	$ 404	$ 466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 342	337	325	315	298	266	227	149	93	$ 27
2012 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	397	397	399	404	394	394	417	414	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 11
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	386	391	407	413	416	431	443	448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	469	465	479	476	479	496	503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 27
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	502	508	516	529	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 54
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	554	555	557	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 89
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	571	577	597	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 124
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	622	641	650
Total of incurred but not reported liabilities plus expected development on reported losses	$ 181
Cumulative number of reported claims | reported_claim	22
Cumulative paid losses and ALAE, net of reinsurance	$ 253	148	$ 44
2019 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	643	672
Total of incurred but not reported liabilities plus expected development on reported losses	$ 293
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 134	$ 39
2020 | Commercial Casualty Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	674
Total of incurred but not reported liabilities plus expected development on reported losses	$ 472
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 33