Property Casualty Loss And Loss Expenses (Claims Development - Commercial Casualty) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2018 USD ($) reported_claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,588
Liabilities for loss and ALAE, net of reinsurance	5,366
Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	4,671
Cumulative paid losses and ALAE, net of reinsurance	2,658
All outstanding liabilities before, net of reinsurance	94
Liabilities for loss and ALAE, net of reinsurance	2,107
2009 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	360	$ 353	$ 357	$ 357	$ 365	$ 370	$ 360	$ 412	$ 409	$ 484
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 338	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	341	343	349	348	347	349	378	394	495
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 318	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	365	366	380	375	377	377	404	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 325	315	298	266	227	149	93	$ 27
2012 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	399	404	394	394	417	414	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 29
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 346	330	288	232	170	88	$ 27
2013 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	407	413	416	431	443	448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 53
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 312	286	232	159	90	$ 35
2014 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	479	476	479	496	503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 66
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 338	287	172	97	$ 34
2015 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	516	529	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 105
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 287	200	108	$ 38
2016 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	557	574	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 152
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 228	126	$ 46
2017 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	597	610
Total of incurred but not reported liabilities plus expected development on reported losses	$ 262
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 122	$ 48
2018 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	650
Total of incurred but not reported liabilities plus expected development on reported losses	$ 406
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 44