Property Casualty Loss And Loss Expenses (Claims Development - Commercial Property) (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 Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	2,353
Cumulative paid losses and ALAE, net of reinsurance	2,095
All outstanding liabilities before, net of reinsurance	12
Liabilities for loss and ALAE, net of reinsurance	270
2013 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	357	$ 356	$ 352	$ 359	$ 389
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 352	349	344	330	$ 227
2014 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	442	441	444	452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 437	432	412	$ 297
2015 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	416	414	454
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 407	388	$ 279
2016 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	551	590
Total of incurred but not reported liabilities plus expected development on reported losses	$ 7
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 504	$ 358
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	587
Total of incurred but not reported liabilities plus expected development on reported losses	$ 24
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 395