Property Casualty Loss And Loss Expenses (Claims Development - Personal 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
Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,774
Cumulative paid losses and ALAE, net of reinsurance	1,503
All outstanding liabilities before, net of reinsurance	5
Liabilities for loss and ALAE, net of reinsurance	276
2013 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	298	$ 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	$ 293	287	272	245	$ 190
2014 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	324	325	317	317
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	105
Cumulative paid losses and ALAE, net of reinsurance	$ 313	298	267	$ 210
2015 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	356	356	343
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	108
Cumulative paid losses and ALAE, net of reinsurance	$ 325	292	$ 229
2016 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	384	383
Total of incurred but not reported liabilities plus expected development on reported losses	$ 8
Cumulative number of reported claims | reported_claim	110
Cumulative paid losses and ALAE, net of reinsurance	$ 316	$ 243
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	412
Total of incurred but not reported liabilities plus expected development on reported losses	$ 55
Cumulative number of reported claims | reported_claim	102
Cumulative paid losses and ALAE, net of reinsurance	$ 256