Claims Development - Commercial Auto (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 ($)
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 Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,235
Cumulative paid losses and ALAE, net of reinsurance	1,581
All outstanding liabilities before, net of reinsurance	26
Liabilities for loss and ALAE, net of reinsurance	680
2016 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	474	$ 463	$ 450	$ 430	$ 417
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	53
Cumulative paid losses and ALAE, net of reinsurance	$ 441	408	350	273	$ 184
2017 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	444	443	441	451
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	442	442	453
Total of incurred but not reported liabilities plus expected development on reported losses	$ 36
Cumulative number of reported claims | reported_claim	49
Cumulative paid losses and ALAE, net of reinsurance	$ 337	266	$ 184
2019 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	451	452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 73
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 268	$ 183
2020 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	424
Total of incurred but not reported liabilities plus expected development on reported losses	$ 164
Cumulative number of reported claims | reported_claim	33
Cumulative paid losses and ALAE, net of reinsurance	$ 154