Property Casualty Loss And Loss Expenses (Claims Development - Personal 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
Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,633
Cumulative paid losses and ALAE, net of reinsurance	1,388
All outstanding liabilities before, net of reinsurance	4
Liabilities for loss and ALAE, net of reinsurance	249
2012 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	271	$ 270	$ 269	$ 265	$ 260
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	89
Cumulative paid losses and ALAE, net of reinsurance	$ 268	262	250	223	$ 174
2013 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	298	293	280	280
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	96
Cumulative paid losses and ALAE, net of reinsurance	$ 287	272	245	$ 190
2014 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	325	317	317
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	104
Cumulative paid losses and ALAE, net of reinsurance	$ 298	267	$ 210
2015 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	356	343
Total of incurred but not reported liabilities plus expected development on reported losses	$ 12
Cumulative number of reported claims | reported_claim	108
Cumulative paid losses and ALAE, net of reinsurance	$ 292	$ 229
2016 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	383
Total of incurred but not reported liabilities plus expected development on reported losses	$ 38
Cumulative number of reported claims | reported_claim	104
Cumulative paid losses and ALAE, net of reinsurance	$ 243