Claims Development - Commercial Auto (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 ($)
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 Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,203
Cumulative paid losses and ALAE, net of reinsurance	1,515
All outstanding liabilities before, net of reinsurance	34
Liabilities for loss and ALAE, net of reinsurance	722
2017 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	449	$ 444	$ 443	$ 441	$ 451
Total of incurred but not reported liabilities plus expected development on reported losses	$ 8
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 411	381	334	266	$ 187
2018 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	440	442	442	453
Total of incurred but not reported liabilities plus expected development on reported losses	$ 15
Cumulative number of reported claims | reported_claim	49
Cumulative paid losses and ALAE, net of reinsurance	$ 378	337	266	$ 184
2019 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	453	451	452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 34
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 333	268	$ 183
2020 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	391	424
Total of incurred but not reported liabilities plus expected development on reported losses	$ 86
Cumulative number of reported claims | reported_claim	36
Cumulative paid losses and ALAE, net of reinsurance	$ 214	$ 154
2021 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	470
Total of incurred but not reported liabilities plus expected development on reported losses	$ 154
Cumulative number of reported claims | reported_claim	36
Cumulative paid losses and ALAE, net of reinsurance	$ 179