Claims Development - Personal 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
Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,860
Cumulative paid losses and ALAE, net of reinsurance	1,610
All outstanding liabilities before, net of reinsurance	10
Liabilities for loss and ALAE, net of reinsurance	260
2016 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	384	$ 384	$ 386	$ 384	$ 383
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	110
Cumulative paid losses and ALAE, net of reinsurance	$ 378	370	351	316	$ 243
2017 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	393	391	394	412
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	109
Cumulative paid losses and ALAE, net of reinsurance	$ 374	358	324	$ 256
2018 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	395	398	424
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	111
Cumulative paid losses and ALAE, net of reinsurance	$ 358	327	$ 262
2019 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	383	399
Total of incurred but not reported liabilities plus expected development on reported losses	$ 14
Cumulative number of reported claims | reported_claim	102
Cumulative paid losses and ALAE, net of reinsurance	$ 314	$ 250
2020 | Personal Auto | Personal Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	305
Total of incurred but not reported liabilities plus expected development on reported losses	$ 62
Cumulative number of reported claims | reported_claim	67
Cumulative paid losses and ALAE, net of reinsurance	$ 186