Claims Development - Commercial Auto (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2019 USD ($) reported_claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 2,739
Liabilities for loss and ALAE, net of reinsurance	5,799
Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,200
Cumulative paid losses and ALAE, net of reinsurance	1,566
All outstanding liabilities before, net of reinsurance	15
Liabilities for loss and ALAE, net of reinsurance	649
2015 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	400	$ 401	$ 394	$ 384	$ 374
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 375	349	303	244	$ 173
2016 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	463	450	430	417
Total of incurred but not reported liabilities plus expected development on reported losses	$ 7
Cumulative number of reported claims | reported_claim	53
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	443	441	451
Total of incurred but not reported liabilities plus expected development on reported losses	$ 27
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 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	453
Total of incurred but not reported liabilities plus expected development on reported losses	$ 72
Cumulative number of reported claims | reported_claim	49
Cumulative paid losses and ALAE, net of reinsurance	$ 266	$ 184
Short-Duration Insurance Contract, Accident Year 2019 | Commercial Auto Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 133
Cumulative number of reported claims | reported_claim	42
Cumulative paid losses and ALAE, net of reinsurance	$ 183