Property Casualty Loss And Loss Expenses (Claims Development - Commercial Casualty) (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 ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,353
Liabilities for loss and ALAE, net of reinsurance	4,930
Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	4,434
Cumulative paid losses and ALAE, net of reinsurance	2,573
All outstanding liabilities before, net of reinsurance	83
Liabilities for loss and ALAE, net of reinsurance	1,944
2008 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	366	$ 355	$ 357	$ 357	$ 359	$ 362	$ 387	$ 409	$ 458	$ 516
Total of incurred but not reported liabilities plus expected development on reported losses	$ 11
Cumulative number of reported claims | reported_claim	0
Cumulative paid losses and ALAE, net of reinsurance	$ 339	331	325	315	293	262	212	158	93	$ 31
2009 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	353	357	357	365	370	360	412	409	484
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 328	322	310	295	267	213	151	75	$ 27
2010 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	343	349	348	347	349	378	394	495
Total of incurred but not reported liabilities plus expected development on reported losses	$ 19
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 314	300	285	256	203	159	92	$ 33
2011 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	366	380	375	377	377	404	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 315	298	266	227	149	93	$ 27
2012 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	404	394	394	417	414	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 43
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 330	288	232	170	88	$ 27
2013 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	413	416	431	443	448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 64
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 286	232	159	90	$ 35
2014 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	476	479	496	503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 82
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 287	172	97	$ 34
2015 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	529	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 170
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 200	108	$ 38
2016 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 247
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 126	$ 46
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 381
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 48