Property Casualty Loss And Loss Expenses (Claims Development - Commercial Auto) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,187
Liabilities for loss and ALAE, net of reinsurance	4,770
Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,768
Cumulative paid losses and ALAE, net of reinsurance	1,264
All outstanding liabilities before, net of reinsurance	12
Liabilities for loss and ALAE, net of reinsurance	516
2012 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	294	$ 288	$ 285	$ 273	$ 275
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	43
Cumulative paid losses and ALAE, net of reinsurance	$ 278	258	222	184	$ 129
2013 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	322	315	305	292
Total of incurred but not reported liabilities plus expected development on reported losses	$ 9
Cumulative number of reported claims | reported_claim	46
Cumulative paid losses and ALAE, net of reinsurance	$ 285	246	199	$ 141
2014 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	351	346	333
Total of incurred but not reported liabilities plus expected development on reported losses	$ 14
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 273	223	$ 159
2015 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	384	374
Total of incurred but not reported liabilities plus expected development on reported losses	$ 38
Cumulative number of reported claims | reported_claim	51
Cumulative paid losses and ALAE, net of reinsurance	$ 244	$ 173
2016 | Commercial Auto Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	417
Total of incurred but not reported liabilities plus expected development on reported losses	$ 81
Cumulative number of reported claims | reported_claim	49
Cumulative paid losses and ALAE, net of reinsurance	$ 184