Claims Development - Commercial Property (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 Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,739
Cumulative paid losses and ALAE, net of reinsurance	2,469
All outstanding liabilities before, net of reinsurance	13
Liabilities for loss and ALAE, net of reinsurance	283
2015 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	414	$ 415	$ 416	$ 414	$ 454
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 411	411	407	388	$ 279
2016 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	545	541	551	590
Total of incurred but not reported liabilities plus expected development on reported losses	$ 4
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 539	528	504	$ 358
2017 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	556	560	587
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 547	522	$ 395
2018 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	603	630
Total of incurred but not reported liabilities plus expected development on reported losses	$ 9
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 559	$ 386
Short-Duration Insurance Contract, Accident Year 2019 | Commercial Property Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	621
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 413