Property Casualty Loss And Loss Expenses (Claims Development - Commercial Auto) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2017 USD ($) reported_claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,353
Liabilities for loss and ALAE, net of reinsurance	4,930
Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,955
Cumulative paid losses and ALAE, net of reinsurance	1,380
All outstanding liabilities before, net of reinsurance	16
Liabilities for loss and ALAE, net of reinsurance	591
2013 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	322	$ 322	$ 315	$ 305	$ 292
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 307	285	246	199	$ 141
2014 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	358	351	346	333
Total of incurred but not reported liabilities plus expected development on reported losses	$ 8
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 310	273	223	$ 159
2015 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	394	384	374
Total of incurred but not reported liabilities plus expected development on reported losses	$ 15
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 303	244	$ 173
2016 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	430	417
Total of incurred but not reported liabilities plus expected development on reported losses	$ 36
Cumulative number of reported claims | reported_claim	53
Cumulative paid losses and ALAE, net of reinsurance	$ 273	$ 184
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	451
Total of incurred but not reported liabilities plus expected development on reported losses	$ 127
Cumulative number of reported claims | reported_claim	47
Cumulative paid losses and ALAE, net of reinsurance	$ 187