Property Casualty Loss And Loss Expenses (Claims Development - Commercial Auto) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2018 USD ($) reported_claim	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,588
Liabilities for loss and ALAE, net of reinsurance	5,366
Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	2,104
Cumulative paid losses and ALAE, net of reinsurance	1,486
All outstanding liabilities before, net of reinsurance	14
Liabilities for loss and ALAE, net of reinsurance	632
2014 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	359	$ 358	$ 351	$ 346	$ 333
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 337	310	273	223	$ 159
2015 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	401	394	384	374
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 349	303	244	$ 173
2016 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	450	430	417
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	53
Cumulative paid losses and ALAE, net of reinsurance	$ 350	273	$ 184
2017 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	441	451
Total of incurred but not reported liabilities plus expected development on reported losses	$ 65
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 266	$ 187
2018 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	453
Total of incurred but not reported liabilities plus expected development on reported losses	$ 138
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 184